gst/avi/gstavidemux.c: Fix position query for audio. also fixes timestamps in streaming mode and bug #364958.

Original commit message from CVS:
* gst/avi/gstavidemux.c: (gst_avi_demux_handle_src_query),
(gst_avi_demux_parse_file_header),
(gst_avi_demux_stream_init_push), (gst_avi_demux_parse_stream),
(gst_avi_demux_stream_header_push), (gst_avi_demux_stream_data),
(gst_avi_demux_chain):
Fix position query for audio. also fixes timestamps in streaming
mode and bug #364958.
Small cleanups.