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>
