Fix uninitialized memory read

Explicitly initialize the markBit to 0. This is important
for stack based execution contexts that can have uninitialized
data in the bit.

Change-Id: I50f1286949f1b4732e3a31b83b238bc7dcf7c7a7
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

src/qml/jsruntime/qv4managed_p.h

@@ -191,6 +191,7 @@ struct Q_QML_PRIVATE_EXPORT Managed
         Data() {}
         Data(InternalClass *internal)
             : internalClass(internal)
+            , markBit(0)
             , inUse(1)
             , extensible(1)
         {