mpegvideo: Initialize chroma_*_shift and codec_tag even if the size is 0

This fixes breakage in a few fate tests on certain setups
(that for some reason didn't break on OS X) after the previous
commit (8812a8057). Currently, some video streams are initialized
in ff_MPV_common_init with width/height set at 0 and only changed
to a proper video size with ff_MPV_common_frame_size_change later.

The breakage was diagnosed by Anton Khirnov.

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 5f24fe82e5fcf227abb5ebf62aa9bc246fda8c0d)

Signed-off-by: Martin Storsjö <martin@martin.st>
h263dec: Remove a hack that can cause infinite loops

The actual usefulness of the hack is not known, and it does cause
infinite loops with some broken input files.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 8812a8057f539845f6801cafdf6c481a59e96b48)

Signed-off-by: Martin Storsjö <martin@martin.st>