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

@@ -853,22 +853,22 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
                             s->suppress_wait_for_response_packet = true;
                             break; /* try something else */
                         }
-                    }
-
-                    /* FIXME: if we ever support variable signature
-                     * flags, this is somewhere they'll need to be
-                     * put */
-                    char *invalid = ssh_key_invalid(key->key, 0);
-                    if (invalid) {
-                        ppl_printf("Cannot use this private key (%s)\r\n",
-                                   invalid);
-                        ssh_key_free(key->key);
-                        sfree(key->comment);
-                        sfree(key);
-                        sfree(invalid);
-                        key = NULL;
-                        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 */
+                        char *invalid = ssh_key_invalid(key->key, 0);
+                        if (invalid) {
+                            ppl_printf("Cannot use this private key (%s)\r\n",
+                                       invalid);
+                            ssh_key_free(key->key);
+                            sfree(key->comment);
+                            sfree(key);
+                            sfree(invalid);
+                            key = NULL;
+                            s->suppress_wait_for_response_packet = true;
+                            break; /* try something else */
+                        }
                     }
                 }