gst/avi/gstavimux.c: First calculate the rate, and only then use it. Hdr.rate is a multiple and not a derivative of h...

Original commit message from CVS:
* gst/avi/gstavimux.c: (gst_avimux_audsinkconnect),
(gst_avimux_stop_file):
First calculate the rate, and only then use it. Hdr.rate is a
multiple and not a derivative of hdr.scale. Scale is not the
same as blockalign but is solely related to rate.