pki: Make sure sig is not used unintialized.

BUG: https://red.libssh.org/issues/167

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

src/pki.c

@@ -1412,7 +1412,7 @@ ssh_string ssh_pki_do_sign(ssh_session session,
     struct ssh_crypto_struct *crypto =
         session->current_crypto ? session->current_crypto :
                                   session->next_crypto;
-    ssh_signature sig;
+    ssh_signature sig = NULL;
     ssh_string sig_blob;
     ssh_string session_id;
     int rc;