vc1dec: Do not use random pred_flag if motion vector data is skipped

This fixes SA10143.vc1 from test-suite. Also partially fixes MC-VC1.ts
from videolan streams archive.

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 082829520e2191625d3c41ed6ad0522e8d27ebe1)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>