Don't use ssize_t for storing difference between sizes

Don't use ssize_t for storing difference between sizes

As POSIX guarantees only that the type ssize_t shall be capable of
storing values at least in the range [-1, {SSIZE_MAX}], it can't be used
to calculate the difference between two memory sizes.  Change the logic
for junk filling to test whether the new size is larger than old size
and then use size_t as the result will be always positive.