v4l2: fix use after free when handling events

The sink_event parent function may consume the event so we shouldn't use
it after having calling it.
16 jobs for <span class="ref-name">v4l2-fix</span> in 43 minutes and 58 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #240595
gst indent

00:00:29

passed #240594
manifest

00:00:37

 
  Build
passed #240596
build fedora x86_64

00:04:49

passed #240597
build nodebug fedora x86_64

00:04:24

 
  Test
passed #240598
check fedora

00:03:04

passed #240599
integration testsuites fedora

00:05:32

 
  Full Builds
passed #240601
build cerbero android universal

00:11:16

passed #240602
build cerbero cross win32

00:07:50

passed #240603
build cerbero cross win64

00:04:33

passed #240600
build cerbero fedora x86_64

00:05:44

 
  Full Tests
passed #240605
valgrind bad

00:06:18

passed #240888
valgrind good

00:09:04

failed #240851
valgrind good

00:04:51

failed #240826
valgrind good

00:15:45

failed #240604
valgrind good

00:07:48

 
  Apps
passed #240606
android universal examples

00:06:19