Windows, compile : reenable previously disabled warning C4291

no matching operator delete found; memory will not be freed if initialization throws an exception

Added a no-op delete() for MEM_ROOT based placement-new()

sql/sql_cursor.h

@@ -62,6 +62,7 @@ public:
   virtual ~Server_side_cursor();
 
   static void operator delete(void *ptr, size_t size);
+  static void operator delete(void *, MEM_ROOT *){}
 };