refs: Fix a logic error

I encountered the Opening remotes/ dir error with some broken pull code, and this fixes it.

Closes: #358
Approved by: cgwalters

src/libostree/ostree-repo-refs.c

@@ -809,14 +809,18 @@ _ostree_repo_write_ref (OstreeRepo    *self,
             goto out;
         }
 
-      if (!glnx_opendirat (refs_remotes_dfd, remote, TRUE, &dfd, error))
+      dfd = glnx_opendirat_with_errno (refs_remotes_dfd, remote, TRUE);
+      if (dfd < 0 && (errno != ENOENT || rev != NULL))
         {
+          glnx_set_error_from_errno (error);
           g_prefix_error (error, "Opening remotes/ dir %s: ", remote);
           goto out;
         }
     }
 
   if (rev == NULL)
+    {
+      if (dfd >= 0)
         {
           if (unlinkat (dfd, ref, 0) != 0)
           {
@@ -827,6 +831,7 @@ _ostree_repo_write_ref (OstreeRepo    *self,
               }
           }
         }
+    }
   else
     {
       if (!write_checksum_file_at (self, dfd, ref, rev, cancellable, error))