Add correct offset for end of chunk

Using `&current->chunk_ + x` will add `x * sizeof(current->chunk_)` to
the pointer, but we only want to add `x`.

Adding a much larger number still "works" on 64bit systems (although it
does not perform the intended check). However on 32bit platforms this
wraps (at least in the way used by sllisttest.cc) and the "end" of the
chunk is suddenly before the beginning.

Closes #57