Fix bounds-checking in policy_summarize

Found by piebeer.

changes/policy_summarize-assert

@@ -0,0 +1,6 @@
+  o Major bugfixes (security)
+    - Fix a bounds-checking error that could allow an attacker to
+      remotely crash a directory authority.  Found by piebeer.
+      Bugfix on 0.2.1.5-alpha.
+
+

src/or/policies.c

@@ -1209,8 +1209,8 @@ policy_summarize(smartlist_t *policy)
   accepts_str = smartlist_join_strings(accepts, ",", 0, &accepts_len);
   rejects_str = smartlist_join_strings(rejects, ",", 0, &rejects_len);
 
-  if (rejects_len > MAX_EXITPOLICY_SUMMARY_LEN &&
-      accepts_len > MAX_EXITPOLICY_SUMMARY_LEN) {
+  if (rejects_len > MAX_EXITPOLICY_SUMMARY_LEN-strlen("reject")-1 &&
+      accepts_len > MAX_EXITPOLICY_SUMMARY_LEN-strlen("accept")-1) {
     char *c;
     shorter_str = accepts_str;
     prefix = "accept";