ffmpeg: Don't try and write sdp info if none of the outputs had an rtp format.

Fixes a segfault when trying to write nonexistent rtp information.
Signed-off-by: Simon Thelen <ffmpeg-dev@c-14.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffmpeg.c

@@ -2447,6 +2447,9 @@ static void print_sdp(void)
         }
     }
 
+    if (!j)
+        goto fail;
+
     av_sdp_create(avc, j, sdp, sizeof(sdp));
 
     if (!sdp_filename) {
@@ -2462,6 +2465,7 @@ static void print_sdp(void)
         }
     }
 
+fail:
     av_freep(&avc);
 }