Fix memory leak if QMetaType::create is called for an unknown type

The memory should be allocated only if we operates on a valid type,
It is a regression introduced by 3d575d4845

Change-Id: Ia31bccd5b41fe090c29df1aeaa69efb706cd25bb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qmetatype.cpp

@@ -1698,8 +1698,9 @@ bool QMetaType::load(QDataStream &stream, int type, void *data)
 void *QMetaType::create(int type, const void *copy)
 {
     QMetaType info(type);
-    int size = info.sizeOf();
-    return info.construct(operator new(size), copy);
+    if (int size = info.sizeOf())
+        return info.construct(operator new(size), copy);
+    return 0;
 }
 
 /*!