QV4::Scoped: Use explicit operator bool

Change-Id: Ibf41d6051fe35aac37fb40ca3c499d0daacebc65
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/qml/jsruntime/qv4scopedvalue_p.h

@@ -406,10 +406,7 @@ struct Scoped
         return getPointer();
     }
 
-    bool operator!() const {
-        return !ptr->m();
-    }
-    operator void *() const {
+    explicit operator bool() const {
         return ptr->m();
     }