Fix the invalid condition variable

Although harmless, the memmove() in tlsdns and tcpdns was guarded by a
current message length variable that was always bigger than 0 instead of
correct current buffer length remainder variable.

lib/isc/netmgr/tcpdns.c

@@ -999,7 +999,7 @@ processbuffer(isc_nmsocket_t *sock) {
 
 	len += 2;
 	sock->buf_len -= len;
-	if (len > 0) {
+	if (sock->buf_len > 0) {
 		memmove(sock->buf, sock->buf + len, sock->buf_len);
 	}