Clear the address when we can't choose a reachable address

When the fascist_firewall_choose_address_ functions don't find a
reachable address, set the returned address to the null address and port.

This is a precautionary measure, because some callers do not check the
return value.

Fixes bug 24736; bugfix on 0.2.8.2-alpha.

changes/bug24736

@@ -0,0 +1,6 @@
+  o Minor bugfixes (address selection):
+    - When the fascist_firewall_choose_address_ functions don't find a
+      reachable address, set the returned address to the null address and port.
+      This is a precautionary measure, because some callers do not check the
+      return value.
+      Fixes bug 24736; bugfix on 0.2.8.2-alpha.

src/or/policies.c

@@ -860,6 +860,8 @@ fascist_firewall_choose_address_base(const tor_addr_t *ipv4_addr,
     ap->port = result->port;
     return 1;
   } else {
+    tor_addr_make_null(&ap->addr, AF_UNSPEC);
+    ap->port = 0;
     return 0;
   }
 }