bugfix: table->get_fields_in_item_tree=true

partitioning was setting subj as a partitioning-specific
hack (GET_FIXED_FIELDS_FLAG field flag to detect partitioning fields)
inside init_lex_with_single_table(). But vcols also use
init_lex_with_single_table(), they run fix_fields()
in open_table_from_share() before partitioning and this messes up
partitioning GET_FIXED_FIELDS_FLAG logic.

sql/sql_lex.cc

@@ -196,7 +196,6 @@ init_lex_with_single_table(THD *thd, TABLE *table, LEX *lex)
   lex->context_analysis_only|= CONTEXT_ANALYSIS_ONLY_VCOL_EXPR;
   select_lex->cur_pos_in_select_list= UNDEF_POS;
   table->map= 1; //To ensure correct calculation of const item
-  table->get_fields_in_item_tree= TRUE;
   table_list->table= table;
   table_list->cacheable_table= false;
   return FALSE;

sql/sql_partition.cc

@@ -936,6 +936,7 @@ static bool fix_fields_part_func(THD *thd, Item* func_expr, TABLE *table,
 
   if (init_lex_with_single_table(thd, table, &lex))
     goto end;
+  table->get_fields_in_item_tree= true;
 
   func_expr->walk(&Item::change_context_processor, 0, &lex.select_lex.context);
   thd->where= "partition function";