v4l2videodec: Fix drain() function return type

Return right type for drain() function.
21 jobs for backport-mr-237-into-1.16-debug in 62 minutes and 44 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Preparation
passed #486049
gst indent

00:00:32

passed #486048
manifest

00:01:04

 
  Build
passed #486224
build cerbero android universal

00:11:55

passed #486225
build cerbero cross win32

00:04:36

passed #486226
build cerbero cross win64

00:05:49

passed #486054
build cerbero fedora x86_64

00:05:36

passed #486059
gst-ios-12.2
build cerbero ios universal

00:36:24

passed #486058
gst-macos-10.14
build cerbero macos x86_64

00:10:22

passed #486050
build fedora x86_64

00:10:27

passed #486051
build nodebug fedora x86_64

00:09:54

passed #486052
build static fedora x86_64

00:12:28

passed #486053
build static nodebug fedora x86_64

00:10:17

failed #486055
build cerbero android universal

00:01:13

failed #486056
build cerbero cross win32

00:01:20

failed #486057
build cerbero cross win64

00:01:22

 
  Test
passed #486060
check fedora

00:03:27

passed #486061
integration testsuites fedora

00:07:50

passed #486063
valgrind bad

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passed #486064
valgrind ges

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passed #486062
valgrind good

00:10:45

 
  Integrate
passed #486065
android universal examples

00:08:17