[tests/buffer] Update tests for previous commit

test/api/test-buffer.c

diff --git a/test/api/test-buffer.c b/test/api/test-buffer.c
index 6904b36..8e82199 100644
--- a/test/api/test-buffer.c
+++ b/test/api/test-buffer.c
@@ -910,12 +910,12 @@
   char test[1024];
   unsigned int num_glyphs, consumed;
   hb_buffer_t *indeterminate = hb_buffer_get_empty ();
-  hb_buffer_serialize (indeterminate, 0, hb_buffer_get_length (indeterminate),
+  hb_buffer_serialize (indeterminate, 0, (unsigned) -1,
     test, sizeof(test), &consumed, NULL, HB_BUFFER_SERIALIZE_FORMAT_JSON,
     HB_BUFFER_SERIALIZE_FLAG_DEFAULT);
   g_assert_cmpstr ( test, ==, "[]");
 
-  hb_buffer_serialize (indeterminate, 0, hb_buffer_get_length (indeterminate),
+  hb_buffer_serialize (indeterminate, 0, (unsigned) - 1,
     test, sizeof(test), &consumed, NULL, HB_BUFFER_SERIALIZE_FORMAT_TEXT,
     HB_BUFFER_SERIALIZE_FLAG_DEFAULT);
   g_assert_cmpstr ( test, ==, "!!");