Fix change-of-sign warning found by ICC

qhostaddress.cpp(263): warning qt#68: integer conversion resulted in a
change of sign
      length = -1;
               ^

I changed the length member from int to quint8 in commit
8656ee9 but I never tested ICC.

Change-Id: I052407b777ec43f78378fffd15311669b490ed7b
Reviewed-by: Timur Pocheptsov <[email protected]>
Reviewed-by: Edward Welbourne <[email protected]>

src/network/kernel/qhostaddress.cpp

@@ -260,7 +260,7 @@ bool QNetmask::setAddress(const QHostAddress &address)
     int netmask = 0;
     quint8 *ptr = ip.v6;
     quint8 *end;
-    length = -1;
+    length = 255;
 
     if (address.protocol() == QAbstractSocket::IPv4Protocol) {
         ip.v4 = qToBigEndian(address.toIPv4Address());