QHostInfo::lookupHost: do not violate lookupHostImpl preconditions

lookupHostImpl requires either `receiver` or `slotObj` to be non-null.
Since this code path deals with the string-based slots, `slotObj` is
going to be null, therefore check that `receiver` is non-null.
For completeness: also check that `member` is non-null.

Emit a warning (à la QObject::connect) in case the user did something
wrong.

Change-Id: Ic6dcd51d7ddd977b121484369b1aef48844364c5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/network/kernel/qhostinfo.cpp

@@ -225,6 +225,10 @@ static int nextId()
 */
 int QHostInfo::lookupHost(const QString &name, QT7_ONLY(const) QObject *receiver, const char *member)
 {
+    if (!receiver || !member) {
+        qWarning("QHostInfo::lookupHost: both the receiver and the member to invoke must be non-null");
+        return -1;
+    }
     return QHostInfo::lookupHostImpl(name, receiver, nullptr, member);
 }