relay: Change LD_BUG log to LD_PROTOCOL_WARN

That log statement can be triggered if somebody on the Internet behaves badly
which is possible with buggy implementation for instance.

Fixes #21293

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/bug21293

@@ -0,0 +1,5 @@
+  o Minor bugfixes (cell, logging):
+    - Downgrade a log statement from bug to protocol warning because there is
+      at least one use case where it can be triggered by a buggy tor
+      implementation on the Internet for instance. Fixes bug 21293; bugfix on
+      tor-0.1.1.14-alpha.

src/or/relay.c

@@ -226,7 +226,8 @@ circuit_receive_relay_cell(cell_t *cell, circuit_t *circ,
     return 0;
 
   if (relay_crypt(circ, cell, cell_direction, &layer_hint, &recognized) < 0) {
-    log_warn(LD_BUG,"relay crypt failed. Dropping connection.");
+    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
+           "relay crypt failed. Dropping connection.");
     return -END_CIRC_REASON_INTERNAL;
   }