gst/mpeg4videoparse/mpeg4videoparse.c: Move some code around to integrate the startco...

gst/mpeg4videoparse/mpeg4videoparse.c: Move some code around to integrate the startcode searching with the other bits...

Original commit message from CVS:
Patch by: Sjoerd Simons <sjoerd at luon dot net>
* gst/mpeg4videoparse/mpeg4videoparse.c: (gst_mpeg4vparse_push),
(gst_mpeg4vparse_drain), (gst_mpeg4vparse_chain),
(gst_mpeg4vparse_change_state):
Move some code around to integrate the startcode searching with the
other bits of parsing, avoid a whole bunch of peeks.
Get rid of invalid data that should not happen according to the specs.
Fixes #533559.