Fix a deadlock when there is a mismatch between stream lengths.

If it is possible to seek to a position in a file where there are no
subsequent video frames but the end of file has not been reached
gst_app_sink_pull_preroll will block forever.  Ensure there is a
frame available before calling.
Merge pull request #74 from adenexter/gstreamer-deadlock

Fix a deadlock when there is a mismatch between stream lengths.