Stop trying to get frames from a video source when an attempt

Stop trying to get frames from a video source when an attempt
to get an earlier frame has already failed because the decoder
said it has no more data.  Before this the VideoDecoder would
repeatedly seek to try to get a frame which does not exist.

This happens when the header of a file is wrong, it would seem;
in the file that triggered the bug the header (as read by DoM or ffprobe)
has a length of 137275 frames but the last frame in the file
(according to DoM or ffprobe -show_frames) is 136207 (44.5s earlier).