transfer: warning: implicit conversion

There is an implicit conversion from "unsigned long" to "long";
rounding, sign extension, or loss of accuracy may result.

Fixed by an added typecast.

lib/transfer.c

@@ -734,7 +734,7 @@ static CURLcode readwrite_data(struct SessionHandle *data,
 
       /* Parse the excess data */
       k->str += nread;
-      nread = excess;
+      nread = (ssize_t)excess;
 
       result = Curl_rtsp_rtp_readwrite(data, conn, &nread, &readmore);
       if(result)