audiodecoder: Fix leak on failed audio gaps

If we fail to process the gap event we need to unref the event or
we end up with a leak.
21 jobs for fix_event_leak_gst_audio_decoder_handle_gap in 54 minutes and 2 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Preparation
passed #382332
gst indent

00:00:45

passed #382331
manifest

00:00:56

 
  Build
passed #382338
build cerbero android universal

00:12:28

passed #382339
build cerbero cross win32

00:10:18

passed #382340
build cerbero cross win64

00:05:52

passed #382337
build cerbero fedora x86_64

00:08:56

manual #382342
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #382341
gst-macos-10.14
build cerbero macos x86_64

00:07:24

passed #382333
build fedora x86_64

00:07:17

passed #382334
build nodebug fedora x86_64

00:06:14

passed #382335
build static fedora x86_64

00:17:30

passed #382336
build static nodebug fedora x86_64

00:10:38

 
  Test
passed #382343
check fedora

00:05:28

passed #382344
integration testsuites fedora

00:13:21

passed #382348
valgrind bad

00:07:06

passed #382345
valgrind base

00:20:02

passed #382349
valgrind ges

00:03:58

passed #382346
valgrind good

00:11:35

passed #382347
valgrind ugly

00:03:05

 
  Integrate
passed #382350
android universal examples

00:06:28

passed #382351
documentation

00:04:21