avformat/rtsp: Use rtsp_st->stream_index

Fixes: out of array access
Fixes: rtpdec_h264.c149/poc

Found-by: Hardik Shah of Vehere
Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rtsp.c

@@ -410,7 +410,7 @@ static void parse_fmtp(AVFormatContext *s, RTSPState *rt,
         if (rtsp_st->sdp_payload_type == payload_type &&
             rtsp_st->dynamic_handler &&
             rtsp_st->dynamic_handler->parse_sdp_a_line) {
-            rtsp_st->dynamic_handler->parse_sdp_a_line(s, i,
+            rtsp_st->dynamic_handler->parse_sdp_a_line(s, rtsp_st->stream_index,
                 rtsp_st->dynamic_protocol_context, line);
         }
     }