avformat/rtsp: use < 0 for error check

Found while reviewing CID1473532 Uninitialized scalar variable

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rtsp.c

@@ -1429,7 +1429,7 @@ retry:
     cur_auth_type = rt->auth_state.auth_type;
     if ((ret = rtsp_send_cmd_with_content_async(s, method, url, header,
                                                 send_content,
-                                                send_content_length)))
+                                                send_content_length)) < 0)
         return ret;
 
     if ((ret = ff_rtsp_read_reply(s, reply, content_ptr, 0, method) ) < 0)