gst/avi/gstavidemux.*: If we have an index, use a duration based on the index instead of blindly trusting the informa...

Original commit message from CVS:
* gst/avi/gstavidemux.c: (gst_avi_demux_handle_src_query),
(gst_avi_demux_handle_src_event), (gst_avi_demux_parse_stream),
(gst_avi_demux_calculate_durations_from_index),
(gst_avi_demux_stream_header):
* gst/avi/gstavidemux.h:
If we have an index, use a duration based on the index instead
of blindly trusting the information in the stream headers
(fixes #331817).