Avoid signed integer overflow by making an addition a subtraction

The task has a very good explanation. The use-case was
  ba.remove(n, INT_MAX);
since you can't pass -1 to the length, and that results in overflow when
you add n+INT_MAX.

Task-number: QTBUG-34694
Change-Id: I365eb86b2d0dabbe0bde67e4e7f33d64fd5793af
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>