Initialize t_self and t_tableOid in statext_expressions_load

The function is building a fake heap tuple, but left some of the header
fields (tid and table OID) uninitialized. Per Coverity report.

Reported-by: Ranier Vilela
Discussion: https://postgr.es/m/CAEudQApj6h8tZ0-eP5Af5PKc5NG1YUc7=SdN_99YoHS51fKa0Q@mail.gmail.com

src/backend/statistics/extended_stats.c

@@ -2420,6 +2420,8 @@ statext_expressions_load(Oid stxoid, int idx)
 
 	/* Build a temporary HeapTuple control structure */
 	tmptup.t_len = HeapTupleHeaderGetDatumLength(td);
+	ItemPointerSetInvalid(&(tmptup.t_self));
+	tmptup.t_tableOid = InvalidOid;
 	tmptup.t_data = td;
 
 	tup = heap_copytuple(&tmptup);