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.

Fix on c03694938ed0, not in any released version of Tor.

changes/bug16115-signing-key-NULL-check

@@ -0,0 +1,6 @@
+  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.

src/or/routerkeys.c

@@ -152,7 +152,8 @@ ed_key_init_from_file(const char *fname, uint32_t flags,
                         ED25519_PUBKEY_LEN)) {
     tor_log(severity, LD_OR, "Cert was for wrong key");
     bad_cert = 1;
-  } else if (tor_cert_checksig(cert, &signing_key->pubkey, now) < 0 &&
+  } else if (signing_key &&
+             tor_cert_checksig(cert, &signing_key->pubkey, now) < 0 &&
              (signing_key || cert->cert_expired)) {
     tor_log(severity, LD_OR, "Can't check certificate");
     bad_cert = 1;