In VC-1 interlaced field pictures, chroma motion vectors can extend beyond
picture boundary even if luma vectors are bounded. The problem shows up
only for hpel interpolated MVs, and may be due to the way motion vectors
are scaled / cropped.

Thanks to Konstantin Shishkov for suggesting the fix. This fixes
long-known segfaults in MC-VC1.ts from videolan streams archive.
Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>