Return from rtp_read when select returns an error

Originally committed as revision 22219 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtpproto.c

@@ -248,6 +248,8 @@ static int rtp_read(URLContext *h, uint8_t *buf, int size)
                 }
                 break;
             }
+        } else if (n < 0) {
+            return AVERROR(EIO);
         }
     }
 #endif