ext/mad/gstmad.c: Fail if caps negotiation fails. Should fix #162184, and should definately be in there regardless of...

Original commit message from CVS:
* ext/mad/gstmad.c: (gst_mad_check_caps_reset), (gst_mad_chain):
Fail if caps negotiation fails. Should fix #162184, and should
definately be in there regardless of it fixing the actual bug.
* gst/avi/gstavimux.c: (gst_avimux_get_type), (gst_avimux_init),
(gst_avimux_write_tag), (gst_avimux_riff_get_avi_header),
(gst_avimux_riff_get_avix_header),
(gst_avimux_riff_get_video_header),
(gst_avimux_riff_get_audio_header), (gst_avimux_write_index),
(gst_avimux_start_file), (gst_avimux_handle_event),
(gst_avimux_change_state):
* gst/avi/gstavimux.h:
Refactor structure writing to use GST_WRITE_UINT macros, add
metadata writing support.