mpegvideo: fix invalid memory access for small video dimensions

When either video dimension is only one macroblock, subtractions
based on v_edge_pos and the macroblock size may be negative. In
that situation, an unsigned comparison isn't sufficent to test for
MV overruns, because a limit of (unsigned)-1 will let any other
value pass.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
