mpeg12: do not fail on zero dimensions in the sequence header.

The total frame size is a combination of the 12 bits in the sequence
header and 2 more bits in the the sequence extension. While the
specification explicitly forbids the dimensions from the sequence header
from being 0 (thus ruling out multiples of 4096), such videos
apparrently exist in the wild so we should attempt to decode them.

Based on a patch by Michael Niedermayer <michaelni@gmx.at>

Fixes Bug 416.