ext/flac/gstflacdec.c: Don't g_assert() where we should just return FALSE; remove unnecessary g_assert(); initialize ...

Original commit message from CVS:
Reviewed by: Tim-Philipp Müller  <tim at centricular dot net>
* ext/flac/gstflacdec.c: (gst_flac_dec_write),
(gst_flac_dec_convert_src), (gst_flac_dec_src_query),
(gst_flac_dec_change_state):
Don't g_assert() where we should just return FALSE; remove
unnecessary g_assert(); initialize some fields properly in
state change function (fixes #325504). Also, use
GST_DEBUG_OBJECT in two more places.