bug #96536: rmlabelinfo() of tape/initlabel.c corrupts memory if called more than once

The rtcp_TransAckn() function needed to reset serrno to 0 before
calling netread_timeout() because if netread_timeout() returned
-1 then whatever was in serrno would be reported as the error
and errno would be ignored.

rtcopy/rtcp_SendRecv.c

@@ -509,6 +509,7 @@ static int rtcp_TransAckn(SOCKET *s,
   if ( data != NULL ) status = *(int *)data;
 
   if ( whereto == ReceiveFrom ) {
+    serrno = 0;
     rc = netread_timeout(*s,hdrbuf,RTCP_HDRBUFSIZ,RTCP_NETTIMEOUT);
     switch (rc) {
     case -1: