queue2: Fix missing/dropped buffering messages at startup

This fixes a bug that occurs when an attempt is made to post a buffering
message before the queue2 was assigned a bus. One common situation where
this happens is when the use-buffering property is set to TRUE before the
queue2 was added to a bin.

If the result of gst_element_post_message() is not checked, and the
aforementioned situation occurs, then last_posted_buffering_percent and
percent_changed will still be updated, as if posting the message succeeded.
Later attempts to post again will not do anything because the code then
assumes that a message with the same percentage was previously posted
successfully and posting again is redundant.

Updating these variables only if posting succeed and explicitely
posting a buffering message in the READY->PAUSED state change ensure that
a buffering message is posted as early as possible.
28 jobs for queue2-fix-dropped-buffering-messages in 98 minutes and 9 seconds (queued for 39 minutes and 58 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #833847
gst indent

00:00:45

passed #833846
manifest

00:00:50

 
  Build
passed #833857
build cerbero cross-android universal

00:18:03

manual #833861
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #833858
build cerbero cross win32

00:08:40

passed #833859
build cerbero cross win64

00:05:45

passed #833856
build cerbero fedora x86_64

00:08:00

passed #833860
gst-macos-10.14
build cerbero macos x86_64

00:07:04

passed #833852
build clang fedora x86_64

00:08:50

passed #833848
build fedora x86_64

00:08:53

passed #833855
docker windows 1607
build msys2

00:14:38

passed #833849
build nodebug fedora x86_64

00:04:25

passed #833850
build static fedora x86_64

00:13:41

passed #833851
build static nodebug fedora x86_64

00:04:44

passed #833853
docker windows 1607
build vs2017 amd64

00:11:09

passed #833854
docker windows 1607
build vs2017 x86

00:11:01

 
  Test
passed #833862
check fedora

00:03:39

failed #834714
integration testsuites fedora

00:50:47

passed #833868
valgrind bad

00:15:26

passed #833865
valgrind base

00:09:26

passed #833864
valgrind core

00:14:15

passed #833869
valgrind ges

00:09:38

passed #833866
valgrind good

00:12:31

passed #833867
valgrind ugly

00:02:15

failed #833863
integration testsuites fedora

00:44:41

 
  Integrate
passed #833870
cross-android universal examples

00:06:01

skipped #833872
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #833871
documentation

00:05:12

 
Name Stage Failure
failed
integration testsuites fedora Test
           Total: 1447
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-logs: found 69272 matching files

Uploading artifacts to coordinator... ok
id=834714 responseStatus=201 Created token=oUU1CtCh
Uploading artifacts...
validate-logs/*.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=834714 responseStatus=201 Created token=oUU1CtCh
ERROR: Job failed: exit code 1