libutils: clearer abort on overflow.

Let's turn a bug into a feature... Since this code is built with intsan,
anyone who caused overflow here will have had an abort, so we know
no-one actually needs the BAD_INDEX return that was presumably the
original author's intent. So let's just mandate that, since it's a lot
harder to ignore an abort than it is to ignore an error return.

Bug: http://b/179044558
Test: treehugger
Change-Id: I08f1018f9da1e09de885699138b7543d55bb2a36