gst-libs/gst/: Turn a few g_assert_not_reached() into g_return_val_if_reached() to avoid compiler warnings (#503930).

Original commit message from CVS:
* gst-libs/gst/cdda/gstcddabasesrc.c: (gst_cdda_base_src_do_seek),
(gst_cdda_base_src_handle_track_seek), (gst_cdda_base_src_create):
* gst-libs/gst/pbutils/install-plugins.c:
(gst_install_plugins_spawn_child), (gst_install_plugins_supported):
* gst-libs/gst/pbutils/missing-plugins.c:
(gst_missing_plugin_message_get_installer_detail),
(gst_missing_encoder_installer_detail_new):
* gst-libs/gst/rtsp/gstrtspconnection.c: (gst_rtsp_connection_send):
* gst-libs/gst/rtsp/gstrtspmessage.c: (gst_rtsp_message_unset):
Turn a few g_assert_not_reached() into g_return_val_if_reached() to
avoid compiler warnings (#503930).