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 52 minutes and 9 seconds (queued for 1 second)
merge request
Name Stage Failure
failed
integration testsuites fedora 3/4 Test
Uploading artifacts...
meson-logs/: found 2 matching files and directories

validate-logs: found 22294 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=18041481 responseStatus=201 Created token=fXbXHAnf
Uploading artifacts...
validate-logs/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=18041481 responseStatus=201 Created token=fXbXHAnf
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
integration testsuites fedora 2/4 Test
Uploading artifacts...
meson-logs/: found 2 matching files and directories

validate-logs: found 22161 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=18041480 responseStatus=201 Created token=gQFACoo4
Uploading artifacts...
validate-logs/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=18041480 responseStatus=201 Created token=gQFACoo4
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
integration testsuites fedora 4/4 Test
Uploading artifacts...
meson-logs/: found 2 matching files and directories

validate-logs: found 21683 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=18041482 responseStatus=201 Created token=crki29aB
Uploading artifacts...
validate-logs/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=18041482 responseStatus=201 Created token=crki29aB
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
integration testsuites fedora 1/4 Test
Uploading artifacts...
meson-logs/: found 2 matching files and directories

validate-logs: found 21821 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=18041479 responseStatus=201 Created token=xrhUYGVL
Uploading artifacts...
validate-logs/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=18041479 responseStatus=201 Created token=xrhUYGVL
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1