Add an assert to QMetaObjectBuilder.

Constructors and destructors don't have a return value, but every other
method return at least "void".

Change-Id: Ie621aff83e44c187e950910d5c0684ba1a6579b8
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qmetaobjectbuilder.cpp

@@ -104,6 +104,7 @@ public:
           attributes(((int)_access) | (((int)_methodType) << 2)),
           revision(_revision)
     {
+        Q_ASSERT((_methodType == QMetaMethod::Constructor) == returnType.isNull());
     }
 
     QByteArray signature;