Fix a leak-on-error case in 0.2.4 spotted by coverity

This one hits if the snprintf() fails when we're writing our IPv6
exit policy. It's new in 0.2.4.7-alpha. Part of bug 7816.

changes/bug7816.024

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - Avoid leaking IPv6 policy content if we fail to format it into
+      a router descriptor. Spotted by Coverity. Fixes part of 7816;
+      bugfix on 0.2.4.7-alpha.

src/or/router.c

@@ -2190,6 +2190,7 @@ router_dump_router_to_string(char *s, size_t maxlen, routerinfo_t *router,
                             "ipv6-policy %s\n", p6);
       if (result<0) {
         log_warn(LD_BUG,"Descriptor printf of policy ran out of room");
+        tor_free(p6);
         return -1;
       }
       written += result;