{video,audio}decoder: avoid bus error when no output frames

There are media with containers exposing streams but without no
output frames. In this case media processing will fail, even if
there are other streams that can be processed, in case of
playbin2, and fire an error in bus at EOS of all streams in
playbin3.

This patch demotes the error to a warning.
27 jobs for dont-break-stream-no-audio in 97 minutes and 5 seconds (queued for 14 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #825544
gst indent

00:00:38

passed #825543
manifest

00:00:43

 
  Build
passed #825554
build cerbero cross-android universal

00:19:27

manual #825558
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #825555
build cerbero cross win32

00:11:49

passed #825556
build cerbero cross win64

00:06:58

passed #825553
build cerbero fedora x86_64

00:07:11

passed #825557
gst-macos-10.14
build cerbero macos x86_64

00:09:12

passed #825549
build clang fedora x86_64

00:07:26

passed #825545
build fedora x86_64

00:05:35

passed #825552
docker windows 1607
build msys2

00:11:16

passed #825546
build nodebug fedora x86_64

00:03:19

passed #825547
build static fedora x86_64

00:12:38

passed #825548
build static nodebug fedora x86_64

00:03:45

passed #825550
docker windows 1607
build vs2017 amd64

00:10:57

passed #825551
docker windows 1607
build vs2017 x86

00:10:04

 
  Test
passed #825559
check fedora

00:03:15

passed #827014
integration testsuites fedora

00:40:25

passed #825564
valgrind bad

00:10:46

passed #825561
valgrind base

00:07:47

passed #825565
valgrind ges

00:11:06

passed #825562
valgrind good

00:25:09

passed #825563
valgrind ugly

00:02:53

failed #825560
integration testsuites fedora

01:00:00

 
  Integrate
passed #825566
cross-android universal examples

00:08:28

manual #825568
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #825567
documentation

00:04:55