repo: Validate checksums have correct length

ostree_checksum_bytes_peek() can return NULL if the checksum has an
incorrect length (most likely from disk corruption) but most callers
are not prepared to handle this and would likely crash.

Use ostree_checksum_bytes_peek_validate() instead, which sets a
GError on an invalid checksum.

src/libostree/ostree-repo-refs.c

@@ -633,10 +633,15 @@ ostree_repo_remote_list_refs (OstreeRepo       *self,
 
           if (ref_name != NULL)
             {
+              const guchar *csum_bytes;
+
               g_variant_get_child (child, 1, "(t@aya{sv})", NULL, &csum_v, NULL);
+              csum_bytes = ostree_checksum_bytes_peek_validate (csum_v, error);
+              if (csum_bytes == NULL)
+                goto out;
+
+              ostree_checksum_inplace_from_bytes (csum_bytes, tmp_checksum);
 
-              ostree_checksum_inplace_from_bytes (ostree_checksum_bytes_peek (csum_v),
-                                                  tmp_checksum);
               g_hash_table_insert (ret_all_refs,
                                    g_strdup (ref_name),
                                    g_strdup (tmp_checksum));