Fix a memory leak in an error case of SAFECOOKIE authentication.

Found by Coverity Scan; fix for CID 507; bugfix on 0.2.3.13-alpha.

changes/memleak_safecookie

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - Fix a small memory leak when trying to decode incorrect base16
+      authenticator during SAFECOOKIE authentication. Found by
+      Coverity Scan. Fixes CID 507. Bugfix on 0.2.3.13-alpha.

src/or/control.c

@@ -3020,6 +3020,7 @@ handle_control_authchallenge(control_connection_t *conn, uint32_t len,
       connection_write_str_to_buf("513 Invalid base16 client nonce",
                                   conn);
       connection_mark_for_close(TO_CONN(conn));
+      tor_free(client_nonce);
       return -1;
     }