ext/mad/gstmad.c: - on half framerate, compute the rate in advance so the comparisons...

ext/mad/gstmad.c: - on half framerate, compute the rate in advance so the comparisons don't compare wrong values

Original commit message from CVS:
* ext/mad/gstmad.c: (gst_mad_check_caps_reset), (gst_mad_chain):
- on half framerate, compute the rate in advance so the comparisons
don't compare wrong values
- don't use mad_synth/frame_mute anymore, this mirrors mad_decoder
behaviour
- don't use mad_header_decode anymore, mad_frame_decode does that
automatically
- when getting rid of consumed bytes, reset the stream's skiplen
(fixes #163867)