Reorder rend_mid_rendezvous() to do protocol violation checks on top.

changes/bug5645

@@ -0,0 +1,5 @@
+  o Code refactoring:
+    - Defensively refactor rend_mid_rendezvous() so that protocol
+      violations and length checks happen in the beginning. Fixes bug
+      5645.
+

src/or/rendmid.c

@@ -276,13 +276,6 @@ rend_mid_rendezvous(or_circuit_t *circ, const uint8_t *request,
   or_circuit_t *rend_circ;
   char hexid[9];
   int reason = END_CIRC_REASON_INTERNAL;
-  base16_encode(hexid,9,(char*)request,request_len<4?request_len:4);
-
-  if (request_len>=4) {
-    log_info(LD_REND,
-             "Got request for rendezvous from circuit %d to cookie %s.",
-             circ->p_circ_id, hexid);
-  }
 
   if (circ->_base.purpose != CIRCUIT_PURPOSE_OR || circ->_base.n_conn) {
     log_info(LD_REND,
@@ -300,6 +293,12 @@ rend_mid_rendezvous(or_circuit_t *circ, const uint8_t *request,
     goto err;
   }
 
+  base16_encode(hexid, 9, (char*)request, 4);
+
+  log_info(LD_REND,
+           "Got request for rendezvous from circuit %d to cookie %s.",
+           circ->p_circ_id, hexid);
+
   rend_circ = circuit_get_rendezvous((char*)request);
   if (!rend_circ) {
     log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,