Fix crash on entering wrong passphrase.

When I added the new call to ssh_key_invalid the other day, I forgot
to avoid calling it if the key is NULL (and therefore even more
obviously invalid).

ssh2userauth.c

@@ -853,8 +853,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
                             s->suppress_wait_for_response_packet = true;
                             break; /* try something else */
                         }
-                    }
-
+                    } else {
                         /* FIXME: if we ever support variable signature
                          * flags, this is somewhere they'll need to be
                          * put */
@@ -871,6 +870,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
                             break; /* try something else */
                         }
                     }
+                }
 
                 if (key) {
                     strbuf *pkblob, *sigdata, *sigblob;