Util: fix binary search exit condition

The loop can exit with an interval of length one or one of
length zero. In the first case it is correct to check which side
of the interval to return, in the second case no comparison should
be made (since there is only one possible value).
In practice, this usually results in one comparison more than needed,
but in some cases (when the position was past the end of the array),
would call the comparator with undefined.

https://bugzilla.gnome.org/show_bug.cgi?id=666614