Add check for syscache lookup failure in update_relispartition().

Omitted in commit 05b38c7e (though it looks like the original blame
belongs to 9e9befac).  A failure is admittedly unlikely, but if it
did happen, SIGSEGV is not the approved method of reporting it.

Per Coverity.  Back-patch to v11 where the broken code originated.

src/backend/commands/indexcmds.c

@@ -3451,10 +3451,11 @@ update_relispartition(Oid relationId, bool newval)
 
 	classRel = table_open(RelationRelationId, RowExclusiveLock);
 	tup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relationId));
+	if (!HeapTupleIsValid(tup))
+		elog(ERROR, "cache lookup failed for relation %u", relationId);
 	Assert(((Form_pg_class) GETSTRUCT(tup))->relispartition != newval);
 	((Form_pg_class) GETSTRUCT(tup))->relispartition = newval;
 	CatalogTupleUpdate(classRel, &tup->t_self, tup);
 	heap_freetuple(tup);
-
 	table_close(classRel, RowExclusiveLock);
 }