Try to avoid crashes if the destructor of a TQMap is called when no private object was previously created

(cherry picked from commit 9974902237)

src/tools/qmap.h

@@ -644,8 +644,12 @@ public:
 #endif
     ~QMap()
     {
-	if ( sh->deref() )
+	if ( sh ) {
-	    delete sh;
+	    if ( sh->deref() ) {
+		delete sh;
+		sh = 0L;
+	    }
+	}
     }
     QMap<Key,T>& operator= ( const QMap<Key,T>& m );
 #ifndef QT_NO_STL