Fix a hard-to-trigger memory leak in launch_resolve

To hit this leak, you need to be a relay that gets a RESOLVE request
or an exit node getting a BEGIN or RESOLVE request.  You must either
have unconfigured (and unconfigurable) nameservers, or you must have
somehow set DisableNetwork after a network request arrived but
before you managed to process it.

So, I doubt this is reached often.  Still, a leak's a leak.  Fix for
bug 5916; bugfix on 0.2.3.9-alpha and 0.1.2.1-alpha.

changes/bug5916

+  o Minor bugfixes:
+    - Fix a memory leak when trying to launch a DNS request when the
+      network is disabled or the nameservers are unconfigurable.  Fix
+      for bug 5916; bugfix on Tor 0.1.2.1-alpha (for the
+      unconfigurable namesrever case) and on 0.2.3.9-alpha (for the
+      DisableNetwork case).

src/or/dns.c

@@ -1389,7 +1389,7 @@ evdns_callback(int result, char type, int count, int ttl, void *addresses,
 static int
 launch_resolve(edge_connection_t *exitconn)
 {
-  char *addr = tor_strdup(exitconn->_base.address);
+  char *addr;
   struct evdns_request *req = NULL;
   tor_addr_t a;
   int r;
@@ -1408,6 +1408,8 @@ launch_resolve(edge_connection_t *exitconn)
     }
   }
 
+  addr = tor_strdup(exitconn->_base.address);
+
   r = tor_addr_parse_PTR_name(
                             &a, exitconn->_base.address, AF_UNSPEC, 0);