It is possible to make the title bars have a zero pixel height while
stacked, by using a blank font and no padding. This causes a division by
zero when attempting to calculate the child index in
container_at_stacked, which then results in a segfault when attempting
to access the child at that bad index (INT_MIN). This just skips the
check to see if the cursor is over a title bar of a child of a stacked
container when the title bar height is zero since there will be no title
bars.