vclptr: assert that we have a >0 ref-count in acquire as well.

Change-Id: Ia2286661a8f62e8a46c44c390ab1a8a5dffe81d0

include/vcl/outdev.hxx

@@ -274,6 +274,7 @@ private:
 
     inline void acquire() const
     {
+        assert(mnRefCnt>0);
         mnRefCnt++;
     }