lib/repo: Fix handling of missing summary files when downloading

The API for downloading a summary file can legitimately return NULL for
the summary file contents when it returns TRUE (success). This indicates
an error 404 — the summary file was not found.

Two call sites were not handling that correctly, which was causing later
assertion failures.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1061

Closes: #1065
Approved by: cgwalters

src/libostree/ostree-repo-pull.c

@@ -4834,6 +4834,14 @@ find_remotes_cb (GObject      *obj,
           g_clear_error (&error);
           continue;
         }
+      else if (summary_bytes == NULL)
+        {
+          g_debug ("%s: Failed to download summary for result ‘%s’. Ignoring. %s",
+                   G_STRFUNC, result->remote->name,
+                   "No summary file exists on server");
+          g_clear_pointer (&g_ptr_array_index (results, i), (GDestroyNotify) ostree_repo_finder_result_free);
+          continue;
+        }
 
       /* Check the metadata in the summary file, especially whether it contains
        * all the @refs we are interested in. */