gst/avi/gstavidemux.c: Don't crash when encountering a stream with an unknown fourcc or codec id. Instead, create a p...

Original commit message from CVS:
* gst/avi/gstavidemux.c: (gst_avi_demux_base_init),
(gst_avi_demux_class_init), (gst_avi_demux_parse_stream),
(gst_avi_demux_change_state):
Don't crash when encountering a stream with an unknown fourcc or
codec id. Instead, create a pad of type video/x-avi-unknown or
audio/x-avi-unknown, which as a side-effect also results in less
confusing error messages in players ('no decoder' vs. 'no streams');
minor fixes to state change function and class_init function.