InlineCheckerTextBuffer: replace some g_asserts by g_returns

To avoid crashes, in and around check_subregion().

On the master branch check_subregion() has anyway been completely
rewritten to have better performances. So no need to track down the
failed assertions.

In adjust_iters() the assertions were excessive, so remove them
altogether.

https://bugzilla.gnome.org/show_bug.cgi?id=778883