Before this commit an av_assert0 would fail if a v4l2 device did not
support a target format.
For example,
./ffmpeg -f v4l2 -codec:v h264 -i /dev/video0 -f mpegts -
would signal an abort if /dev/video0 did not support h264.

The new behaviour is to return an AVERROR(EINVAL) error code. An
av_assert0 has been added to verify this return.

Fixes #6629
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>