ffmpeg: avoid endlessly reading from one input while another is unavailable

Testcase in Ticket 4051

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/tiffenc: properly compute packet size

The bytes per row is a better indication of it.

Helps resolving ticket #3874 by fixing ffmpeg's encoder and transforming
the issue in a issue with non-compliant decoders. ffmpeg's one is ok,
but unfortunately, many others aren't handling correctly unusual chroma
samplings.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>