Patch for: T22104

When an AVI file contains null frames, ffmpeg skips to the next non-null frame without returning the intermediate frames. This results in the internal position of a movie clip and the actual stream position being unsynchronized, as the internal position is assumed to be monotonically increasing by 1.

See: http://www.virtualdub.org/blog/pivot/entry.php?id=103 for a description of the problem.

This patch will trigger a seek if the presentation timestamp of the frame read isn't the one that we expected. It fixes bug #22104.