smalloc: rework double free bug fix

Rework the fix from commit 6810132 in a way that removes ~60 lines of
code.

The bug was introduced in commit e87ceb2 (mea culpa) and is at its core
a pointer aliasing bug where sometimes two independent pointers existed
that pointed to the same chunk of heap memory.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>