fix a pair of lurking segfaults

svn:r1198

src/or/circuit.c

@@ -65,8 +65,11 @@ void circuit_close_all_marked()
     global_circuitlist = tmp;
   }
 
+  if(!global_circuitlist)
+    return;
+
   for (tmp = global_circuitlist; tmp->next; tmp=tmp->next) {
-    while (tmp->next->marked_for_close) {
+    while (tmp->next && tmp->next->marked_for_close) {
       m = tmp->next->next;
       circuit_free(tmp->next);
       tmp->next = m;