muxmp4: compute audio frame durations when the frame size isn't known.

Otherwise, mp4v2 would assume a duration of zero and never write any audio chunk, causing excessive memory usage and a noticeable performance decrease in some cases.

The issue was introduced in SVN revision 4825, when libavcodec stopped setting AVCodecContext.frame_size for some codecs.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5408 b64f7644-9d1e-0410-96f1-a4d463321fa5