libhb: fix duration of initial audio frame when decoding with avcodec
For some codecs, the samplerate is not known until we decode some data.
So postpone calculation of frame duration till after we have decoded
some data.

The effect of this error is that the first 2 audio frames could have the
same start time.  This causes the "non monotonically increasing dts"
error in libavformat mp4 muxer.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5849 b64f7644-9d1e-0410-96f1-a4d463321fa5