libavcodec: Don't crash in avcodec_encode_audio if time_base isn't set

Earlier, calling avcodec_encode_audio worked fine even if time_base
wasn't set. Now it crashes due to trying to scale the output pts to
the codec context time base. This affects e.g. VLC.

If no time_base is set for audio codecs, set it to the sample
rate.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
