fixed bug, which lead to crash mysqld by running 'mysql some_db_with_tables'

sql/item.cc

@@ -50,7 +50,13 @@ Item::Item():
   next= thd->free_list;			// Put in free list
   thd->free_list= this;
   loop_id= 0;
-  if (thd->lex.current_select->parsing_place == SELECT_LEX_NODE::SELECT_LIST)
+  /*
+    Item constractor can be called during execution other tnec SQL_COM
+    command => we should check thd->lex.current_select on zero (thd->lex
+    can be uninitialized)
+  */
+  if (thd->lex.current_select &&
+      thd->lex.current_select->parsing_place == SELECT_LEX_NODE::SELECT_LIST)
     thd->lex.current_select->select_items++;
 }