Demote "Invalid length on ESTABLISH_RENDEZVOUS" to protocol_warn

Fixes bug 11279

changes/bug11279

@@ -0,0 +1,3 @@
+  o Minor features:
+    - Warn less verbosely when receiving a misformed ESTABLISH_RENDEZVOUS
+      cell. Fixes ticket 11279.

src/or/rendmid.c

@@ -231,7 +231,8 @@ rend_mid_establish_rendezvous(or_circuit_t *circ, const uint8_t *request,
   }
 
   if (request_len != REND_COOKIE_LEN) {
-    log_warn(LD_PROTOCOL, "Invalid length on ESTABLISH_RENDEZVOUS.");
+    log_fn(LOG_PROTOCOL_WARN,
+           LD_PROTOCOL, "Invalid length on ESTABLISH_RENDEZVOUS.");
     goto err;
   }