Fix CONFIG_DELETE crash

Closing a connection can abandon its ops without going thru the threadpool,
so make sure to lock the si_ops_mutex before walking the list.

servers/slapd/overlays/syncprov.c

@@ -3172,6 +3172,7 @@ syncprov_db_close(
 
 #ifdef SLAP_CONFIG_DELETE
 	if ( !slapd_shutdown ) {
+		ldap_pvt_thread_mutex_lock( &si->si_ops_mutex );
 		for ( so=si->si_ops, sonext=so;  so; so=sonext  ) {
 			SlapReply rs = {REP_RESULT};
 			rs.sr_err = LDAP_UNAVAILABLE;
@@ -3180,6 +3181,7 @@ syncprov_db_close(
 			syncprov_drop_psearch( so, 0);
 		}
 		si->si_ops=NULL;
+		ldap_pvt_thread_mutex_unlock( &si->si_ops_mutex );
 	}
 	overlay_unregister_control( be, LDAP_CONTROL_SYNC );
 #endif /* SLAP_CONFIG_DELETE */