ffmpeg: Fix regression where various fields where not initialized leading to segfaults

Bug introduced by:
commit f12f40b3
Author: Anton Khirnov <anton@khirnov.net>
Date:   Tue Aug 30 04:05:20 2011 +0200

    ffmpeg: get rid of new* options.

    They are confusing, irregular and redundant -- -map already contains all
    the information.  Stream maps can now be parsed in opt_output_file().

    Add a more user-friendly default behavior in case no maps are present.

    Breaks -programid for now, but it never worked properly anyway. A better
    solution will be written soon.