Verify that heap tuple is valid before using

If the heap tuple fetched from the cache in
`ts_bgw_job_validate_job_owner` is not valid, the error will print
garbage for the role name, so check that the fetched tuple is valid
before using it.

src/bgw/job.c

@@ -961,6 +961,10 @@ void
 ts_bgw_job_validate_job_owner(Oid owner)
 {
 	HeapTuple role_tup = SearchSysCache1(AUTHOID, ObjectIdGetDatum(owner));
+
+	if (!HeapTupleIsValid(role_tup))
+		elog(ERROR, "cache lookup failed for role %u", owner);
+
 	Form_pg_authid rform = (Form_pg_authid) GETSTRUCT(role_tup);
 
 	if (!rform->rolcanlogin)