ffserver: Clear avio context after closing it in rtp_new_av_stream()

Suggested-by: "Reynaldo H. Verdejo Pinochet" <reynaldo@osg.samsung.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffserver.c

@@ -3413,6 +3413,7 @@ static int rtp_new_av_stream(HTTPContext *c,
         return -1;
     }
     avio_close_dyn_buf(ctx->pb, &dummy_buf);
+    ctx->pb = NULL;
     av_free(dummy_buf);
 
     c->rtp_ctx[stream_index] = ctx;