rtph264depay: fix caps leak

Exposed by rtp_h264depay_bytestream() unit test.
8 jobs for rtph264depay-caps-leak in 48 minutes and 35 seconds (queued for 26 minutes and 57 seconds)
Status Job ID Name Coverage
  Preparation
passed #156744
gst indent

00:00:27

passed #156743
manifest

00:01:06

 
  Build
passed #156745
build fedora x86_64

00:03:53

 
  Test
passed #156746
check fedora

00:04:45

 
  Full Builds
passed #156747
build android arm64 api28

00:05:56

passed #156749
build cerbero cross win32

00:12:18

passed #156750
build cerbero cross win64

00:10:10

passed #156748
build cerbero fedora x86_64

00:09:56