qtdemux: fix leak in Opus caps

The caps for Opus is generated via a utility function that creates a new
GstCaps instance after validating the codec arguments. There are possible ways
to solve this, we chose the one requiring the least number of modifications.

It would be also possible to simply set the caps with the
specified arguments; however, this will required to move all the validation
to qtdemux or require a change to gstreamer codec-utils.
20 jobs for qtdemux-opus in 42 minutes and 31 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #364336
gst indent

00:00:34

passed #364335
manifest

00:00:43

 
  Build
passed #364342
build cerbero android universal

00:11:19

passed #364343
build cerbero cross win32

00:09:55

passed #364344
build cerbero cross win64

00:07:35

passed #364341
build cerbero fedora x86_64

00:07:02

manual #364346
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #364653
gst-macos-10.14
build cerbero macos x86_64

00:06:36

passed #364337
build fedora x86_64

00:07:30

passed #364338
build nodebug fedora x86_64

00:05:19

passed #364339
build static fedora x86_64

00:10:27

passed #364340
build static nodebug fedora x86_64

00:12:08

failed #364345
gst-macos-10.14
build cerbero macos x86_64

02:24:45

 
  Test
passed #364347
check fedora

00:03:51

passed #364348
integration testsuites fedora

00:08:30

passed #364350
valgrind bad

00:04:26

passed #364351
valgrind ges

00:03:43

passed #364349
valgrind good

00:16:16

 
  Integrate
passed #364352
android universal examples

00:06:45

passed #364353
documentation

00:04:15