CVE-2025-4878 legacy: Properly check return value to avoid NULL pointer dereference

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/legacy.c

@@ -441,7 +441,7 @@ ssh_private_key privatekey_from_file(ssh_session session,
                                      auth_fn,
                                      auth_data,
                                      &key);
-    if (rc == SSH_ERROR) {
+    if (rc != SSH_OK) {
         return NULL;
     }