gpg: Initialize a variable even in a never used code path.

* g10/sign.c (write_signature_packets): Init ERR.
--

Actually we could also remove the conditional or replace it by a
log_assert.

GnuPG-bug-id: 5204

g10/sign.c

@@ -870,6 +870,8 @@ write_signature_packets (ctrl_t ctrl,
           else
             err = 0;
         }
+      else
+        err = 0;  /* Actually never reached.  */
       hash_sigversion_to_magic (md, sig);
       gcry_md_final (md);