avtpcvfdepay: Don't hide gst_pad_push return

avtpcvfdepay was effectively hiding any return from gst_pad_push, so no
errors or GST_FLOW_EOS would be propagated upstream.

Tests also added.
23 jobs for cvf-fixes in 46 minutes and 3 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #899804
gst indent

00:00:26

passed #899803
manifest

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  Build
passed #899814
build cerbero cross-android universal

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manual #899818
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
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build cerbero cross win32

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passed #899816
build cerbero cross win64

00:09:31

passed #899813
build cerbero fedora x86_64

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passed #899817
gst-macos-10.14
build cerbero macos x86_64

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passed #899809
build clang fedora x86_64

00:12:08

passed #899805
build fedora x86_64

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passed #899812
docker windows 1607
build msys2

00:11:51

passed #899806
build nodebug fedora x86_64

00:04:55

passed #899807
build static fedora x86_64

00:09:19

passed #899808
build static nodebug fedora x86_64

00:03:08

passed #899810
docker windows 1607
build vs2017 amd64

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passed #899811
docker windows 1607
build vs2017 x86

00:09:49

 
  Test
passed #899852
check fedora

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passed #899820
integration testsuites fedora

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passed #899821
valgrind bad

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failed #899819
check fedora

00:03:40

 
  Integrate
passed #899822
cross-android universal examples

00:06:52

manual #899824
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #899823
documentation

00:04:39