QSharedPointer: destroy LHS on move assignment

Howard Hinnant is right: just swapping may keep a resource alive too long.
The problem with replacing swap(other) was that the naïve approach:

    clear(); swap(other);

is not safe for self-assignment. Taking a cue from the default std::swap()
implementation, and the copy-swap idiom, a self-assignment-safe version is

    QSharedPointer moved(std::move(other)); swap(moved);

which is to what I changed the implementation now.

Change-Id: I589fdae50ae22b95350db8250b02d983dc8487a6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qsharedpointer_impl.h

@@ -332,7 +332,8 @@ public:
     }
     inline QSharedPointer &operator=(QSharedPointer &&other)
     {
-        swap(other);
+        QSharedPointer moved(std::move(other));
+        swap(moved);
         return *this;
     }
 #endif