Fix audio does not play for audio only file

[bmatherly]

This specific file was a WMA file with DTS coded audio. The first packet of audio does not have a PTS of zero. Since there is no video in the file, MLT tries to seek to position 0 for the first audio frame and attempts to sync audio frames as if they started at 0. This patch finds a first_pts value if there is no video in the file to look for.

A user sent me this test file privately. If a release is eminent, I suggest waiting until after the release to merge this change.