Fix memory leak in safe-cookie authentication code

Coverity spotted this. Bug 7816. Fix on 0.2.3.13-alpha.

changes/bug7816_023

@@ -0,0 +1,3 @@
+  o Minor bugfixes (memory leak, controller):
+    - Fix a memory leak during safe-cookie controller authentication.
+      Spotted by Coverity. Fixes part of bug 7816; bugfix on 0.2.3.13-alpha.

src/or/control.c

@@ -3099,6 +3099,8 @@ handle_control_authchallenge(control_connection_t *conn, uint32_t len,
                            "SERVERNONCE=%s\r\n",
                            server_hash_encoded,
                            server_nonce_encoded);
+
+  tor_free(client_nonce);
   return 0;
 }