[release] debugallocator: use unsigned integer type for pointer arithmetic

ptrdiff_t is a signed integer type and so the expression
  (std::uintptr_t)(ptr) % page_size
could become a negative value. In this case the page_ptr would be the
address of the next page after the allocation.

This wrong behaviour could be observed on (32bit) PowerPC: here
ptr was 0xf78cfe00 and page_ptr was calculated as 0xf78d0000 instead
of the correct 0xf78c0000.

(cherry picked from commit 1659ea4b)
Signed-off-by: Carsten Gräser <graeser@dune-project.org>