avformat/matroskadec: correct spurious keyframe warnings in webm

Minor change to fix for Trac #1003 to avoid squawking about files
with valid keyframes that aren't marked (as opposed to frames that are
marked as keyframes and aren't).

Change correctly surpresses the spurious warnings, while still complaining
about (and handling) the genuinely broken file attached to bug #1003.

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