Fixed warning from UBSAN

The warning comes from copying POSITION objects where 'type' is not
initialized.  This does not affect any production code as when 'type'
was compared/used, it was always initialized.

Removed by initializing the type variable in the constructor

sql/sql_select.cc

@@ -391,6 +391,7 @@ POSITION::POSITION()
   range_rowid_filter_info= 0;
   ref_depend_map= dups_producing_tables= 0;
   inner_tables_handled_with_other_sjs= 0;
+  type= JT_UNKNOWN;
   dups_weedout_picker.set_empty();
   firstmatch_picker.set_empty();
   loosescan_picker.set_empty();