Setting Field::field_index for Virtual_tmp_table fields

Virtial_tmp_table did not set the "field_index" member for its Fields.
Fixing Virtual_tmp_table::add() to set "field_index" to the Field's ordinal position
inside the table, like a normal TABLE does, for consistency.

Although, this flaw did not seem to cause any bugs, having field_index properly
set is helpful for debugging purposes.

sql/sql_select.h

@@ -2104,7 +2104,7 @@ public:
       DBUG_ASSERT(s->blob_fields <= m_alloced_field_count);
       s->blob_field[s->blob_fields - 1]= s->fields;
     }
-    s->fields++;
+    new_field->field_index= s->fields++;
     return false;
   }