ext/flac/gstflacdec.c: Fix 'xyz may be used uninitialized' compiler warnings caused by broken g_assert_not_reached() ...

Original commit message from CVS:
* ext/flac/gstflacdec.c: (gst_flac_dec_write):
Fix 'xyz may be used uninitialized' compiler warnings caused
by broken g_assert_not_reached() macro in GLib-2.15.x and don't
abort() in any case but properly report the error.