Treat null address as "unknown", not "rejected" in md policy

Previously, we had an issue where we'd treat an unknown address as
0, which turned into "0.0.0.0", which looked like a rejected
address.  This meant in practice that as soon as we started doing
comparisons of unknown uint32 addresses to short policies, we'd get
'rejected' right away.  Because of the circumstances under which
this would be called, it would only happen when we had local DNS
cached entries and we were looking to launch new circuits.

changes/shortpolicy_compare

@@ -0,0 +1,4 @@
+  o Major bugfixes:
+    - Fix a bug where comparing an unknown address to a microdescriptor's
+      shortened exit policy would always seem to give a "rejected" result.
+      Bugfix on 0.2.3.1-alpha. Fixes bug 3599.

src/or/policies.c

@@ -1415,8 +1415,10 @@ compare_tor_addr_to_short_policy(const tor_addr_t *addr, uint16_t port,
 
   tor_assert(port != 0);
 
+  if (addr && tor_addr_is_null(addr))
+    addr = NULL; /* Unspec means 'no address at all,' in this context. */
+
   if (addr && (tor_addr_is_internal(addr, 0) ||
-               tor_addr_is_null(addr) ||
                tor_addr_is_loopback(addr)))
     return ADDR_POLICY_REJECTED;