Bug 630097 - (cov#11933) Fix NULL dereference in schema code

There is a possibility of deferencing prevocp when it is NULL
the second time through the loop if the first pass was not a
standard objectclass definition and tmpocp != curlisthead.

I don't think that this issue is possible unless some other
thread was able to modify tmpocp->oc_next between where curlisthead
is set (schema.c:2654) and where nextocp is set (schema.c:2658) the
first time through the loop. That said, I see no harm in checking
if prevocp is NULL before attempting to dereference it.

ldap/servers/slapd/schema.c

@@ -2653,7 +2653,9 @@ clean_up_and_return:
 				if ( tmpocp == curlisthead ) {
 					curlisthead = tmpocp->oc_next;
 				} else {
-					prevocp->oc_next = tmpocp->oc_next;
+					if (prevocp) {
+						prevocp->oc_next = tmpocp->oc_next;
+					}
 				}
 				nextocp = tmpocp->oc_next;
 				oc_free( &tmpocp );