Fix 'xyz may be used uninitialized' compiler warnings caused by broken g_assert_not_reached() macro in GLib-2.15.x an...

Original commit message from CVS:
* ext/ladspa/gstladspa.c: (gst_ladspa_get_property):
* ext/sdl/sdlvideosink.c: (gst_sdlvideosink_show_frame):
* gst/mve/gstmvemux.c: (gst_mve_mux_request_new_pad):
* sys/dvb/dvbbasebin.c: (dvb_base_bin_class_init):
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.