avformat/swfenc: Fix memleak upon write_header error

avformat/swfenc: Fix memleak upon write_header error

The SWF muxer accepts at most one mp3 audio and at most one VP6F, FLV1
or MJPEG stream. Upon encountering an mp3 stream, a fifo is allocated
that leaks if one of the subsequent streams is incompliant with the
restrictions mentioned above or if the framerate or samplerate are
invalid. This is fixed by adding a deinit function to free said fifo.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>