fix segv, fmt_ctx is not set for rtsp

Originally committed as revision 13910 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffserver.c

@@ -2206,7 +2206,7 @@ static int http_prepare_data(HTTPContext *c)
                     }
                     ost = ctx->streams[pkt.stream_index];
 
-                    c->fmt_ctx.pb->is_streamed = 1;
+                    ctx->pb->is_streamed = 1;
                     if (pkt.dts != AV_NOPTS_VALUE)
                         pkt.dts = av_rescale_q(pkt.dts, ist->time_base, ost->time_base);
                     if (pkt.pts != AV_NOPTS_VALUE)