gst/avi/gstavidemux.c: Fix index creation when we have to scan the file to create...

gst/avi/gstavidemux.c: Fix index creation when we have to scan the file to create an index. There may be other types ...

Original commit message from CVS:
Patch by: Fabrizio Gennari  <fabrizio dot ge at tiscali dot it>
* gst/avi/gstavidemux.c: (gst_avi_demux_peek_tag),
(gst_avi_demux_next_data_buffer), (gst_avi_demux_stream_scan):
Fix index creation when we have to scan the file to create
an index. There may be other types of RIFF 'LIST' chunks than
'movi' and we need to skip them properly as well or we'll end up
reading garbage (#336889). Some other cosmetic changes.