avformat/rtsp: add av_assert0() to clarify that the else case is supposed to be unreachable

If its reachable then theres a bug as err would be uninitialized
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtsp.c

@@ -1797,6 +1797,8 @@ redirect:
         err = ff_rtsp_setup_input_streams(s, reply);
     else if (CONFIG_RTSP_MUXER)
         err = ff_rtsp_setup_output_streams(s, host);
+    else
+        av_assert0(0);
     if (err)
         goto fail;