tor/changes/bug16115-signing-key-NULL-check

  o Minor fixes (threads, comments):
    - Ensure signing_key is non-NULL before accessing one of its members
      signing_key can be NULL in ed_key_init_from_file in routerkeys.c.
      Discovered by clang 3.7 address sanitizer.
      Patch by "teor".
      Fix on c03694938ed0, not in any released version of Tor.