gst/qtdemux/qtdemux.c: Remove some asserts and replace them with a proper error message. Fixes #379261.

Original commit message from CVS:
* gst/qtdemux/qtdemux.c: (gst_qtdemux_get_duration),
(gst_qtdemux_handle_src_query), (qtdemux_parse_trak):
Remove some asserts and replace them with a proper error
message. Fixes #379261.