fixed Bug #4973 Memory is not released when HEAP table is dropped

sql/ha_heap.cc:
  added calling fn_format(name,..) for name before heap_delete_table
  as it's done before heap_create
  
  fixed Bug #4973 Memory is not released when HEAP table is dropped

sql/ha_heap.cc

@@ -264,7 +264,8 @@ THR_LOCK_DATA **ha_heap::store_lock(THD *thd,
 
 int ha_heap::delete_table(const char *name)
 {
-  int error=heap_delete_table(name);
+  char buff[FN_REFLEN];
+  int error= heap_delete_table(fn_format(buff,name,"","",4+2));
   return error == ENOENT ? 0 : error;
 }