rtp: opuspay: fix memory leak in gst_rtp_opus_pay_setcaps.

The src caps were never dereferenced, causing a memory leak.
18 jobs for backport-mr-271-into-1.16-rtp_opuspay_memleak_fix in 71 minutes and 33 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Preparation
passed #486343
gst indent

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passed #486342
manifest

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  Build
passed #486349
build cerbero android universal

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build cerbero cross win32

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passed #486351
build cerbero cross win64

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passed #486348
build cerbero fedora x86_64

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passed #486353
gst-ios-12.2
build cerbero ios universal

00:33:27

passed #486352
gst-macos-10.14
build cerbero macos x86_64

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passed #486344
build fedora x86_64

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passed #486345
build nodebug fedora x86_64

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passed #486346
build static fedora x86_64

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passed #486347
build static nodebug fedora x86_64

00:11:27

 
  Test
passed #486354
check fedora

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passed #486355
integration testsuites fedora

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passed #486357
valgrind bad

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passed #486358
valgrind ges

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passed #486356
valgrind good

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  Integrate
passed #486359
android universal examples

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