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_show.cc

@@ -2699,6 +2699,7 @@ public:
   static void operator delete(void *ptr __attribute__((unused)),
                               size_t size __attribute__((unused)))
   { TRASH(ptr, size); }
+  static void operator delete(void *, MEM_ROOT *){}
 
   my_thread_id thread_id;
   uint32 os_thread_id;