decodebin3: fix collection leaks

- gst_message_parse_stream_collection() returns an owned collection.
- get_merged_collection() returns one as well and was leaked in the
  else block.

Fix leak when running:
GST_TRACERS=leaks GST_DEBUG="GST_TRACER:7,leaks:6" gst-play-1.0 --use-playbin3 test.mkv

Part-of: <gstreamer/gstreamer!954>
24 jobs for !954 with gst-plugins-base-playbin-leak in 63 minutes and 13 seconds (queued for 13 seconds)
latest detached
Status Name Job ID Coverage
  Build Docker
passed fedora amd64 docker #14205421

00:02:09

passed gst-indent amd64 docker #14205422

00:00:54

passed windows amd64 docker #14205423
windows shell 1809

00:00:21

 
  Preparation
passed check allow-edit #14205425

00:01:22

passed gst indent #14205424
manual

00:00:21

 
  Build
passed build clang fedora x86_64 #14205429

00:05:47

passed build documentation #14205433

00:09:01

manual build msys2 #14205432
docker windows 1809 allowed to fail manual
passed build nodebug fedora x86_64 #14205426

00:03:59

passed build static fedora x86_64 #14205427

00:03:36

passed build static nodebug fedora x86_64 #14205428

00:03:12

passed build vs2017 amd64 #14205430
docker windows 1809

00:10:08

passed build vs2017 x86 #14205431
docker windows 1809

00:10:20

failed cerbero trigger #14205434

01:00:01

 
  Test
passed check fedora #14205435
gstreamer

00:08:06

failed integration testsuites fedora 1/4 #14205436
gstreamer

00:09:13

failed integration testsuites fedora 2/4 #14205437
gstreamer

00:13:45

failed integration testsuites fedora 3/4 #14205438
gstreamer

00:11:33

failed integration testsuites fedora 4/4 #14205439
gstreamer

00:15:44

passed valgrind bad #14205443
gstreamer

00:10:27

passed valgrind base #14205440
gstreamer

00:10:15

passed valgrind ges #14205444
gstreamer

00:08:18

passed valgrind good #14205441
gstreamer

00:09:33

passed valgrind ugly #14205442
gstreamer

00:06:59

 
Name Stage Failure
failed
cerbero trigger Build The script exceeded the maximum execution time set for the job
  Downloading https://files.pythonhosted.org/packages/3f/65/69e6754102dcd018a0f29e4db673372eb323ee504431125ab6c9109cb21c/charset_normalizer-2.0.6-py3-none-any.whl
Installing collected packages: certifi, urllib3, charset-normalizer, requests, requests-toolbelt, python-gitlab
WARNING: The script normalizer is installed in '/root/.local/bin' which is not on PATH.
Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
WARNING: The script gitlab is installed in '/root/.local/bin' which is not on PATH.
Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
Successfully installed certifi-2021.5.30 charset-normalizer-2.0.6 python-gitlab-2.10.1 requests-2.26.0 requests-toolbelt-0.9.1 urllib3-1.26.7
$ ci/gitlab/trigger_cerbero_pipeline.py
Cerbero pipeline running at https://gitlab.freedesktop.org/gstreamer/cerbero/-/pipelines/412954 .............................................................................................................................................................................................................................................
ERROR: Job failed: execution took longer than 1h0m0s seconds
failed
integration testsuites fedora 4/4 Test
Uploading artifacts for failed job
Uploading artifacts...
meson-logs/: found 2 matching files and directories

validate-logs: found 21248 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=14205439 responseStatus=201 Created token=PWZqyBf9
Uploading artifacts...
validate-logs/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=14205439 responseStatus=201 Created token=PWZqyBf9
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
integration testsuites fedora 3/4 Test
Uploading artifacts for failed job
Uploading artifacts...
meson-logs/: found 2 matching files and directories

validate-logs: found 22042 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=14205438 responseStatus=201 Created token=jo2XXSdR
Uploading artifacts...
validate-logs/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=14205438 responseStatus=201 Created token=jo2XXSdR
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
integration testsuites fedora 1/4 Test
Uploading artifacts for failed job
Uploading artifacts...
meson-logs/: found 2 matching files and directories

validate-logs: found 21131 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=14205436 responseStatus=201 Created token=tj1zxcvZ
Uploading artifacts...
validate-logs/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=14205436 responseStatus=201 Created token=tj1zxcvZ
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
integration testsuites fedora 2/4 Test
Uploading artifacts for failed job
Uploading artifacts...
meson-logs/: found 2 matching files and directories

validate-logs: found 21797 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=14205437 responseStatus=201 Created token=1DaZxRuv
Uploading artifacts...
validate-logs/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=14205437 responseStatus=201 Created token=1DaZxRuv
Cleaning up file based variables
ERROR: Job failed: exit code 1