tests: Add a missing assertion

This fixes a warning about a dead store to `newpath` from scan-build.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1767

glib/tests/fileutils.c

@@ -1856,6 +1856,7 @@ test_read_link (void)
       g_test_expect_message (G_LOG_DOMAIN, G_LOG_LEVEL_CRITICAL,
                              "*assertion*!= NULL*");
       newpath = g_file_read_link (NULL, &error);
+      g_assert_null (newpath);
       g_test_assert_expected_messages ();
     }