Avoid out-of-bound pointers and integer overflows in size comparisons

Avoid out-of-bound pointers and integer overflows in size comparisons

This changes pointer calculations in size comparions to a form that
ensures that no out-of-bound pointers are computed, because even their
computation yields undefined behavior.
Also, this changes size comparions to a form that ensures that neither
the left-hand side nor the right-hand side can overflow.