smalloc: fix garbage collection problem

smalloc: fix garbage collection problem

If a large request arrives when pool->next_non_full points to empty
space that is insufficient to satisfy the request, pool->next_non_full
will be inappropriately advanced when the free space is followed by
lines of fully allocated space. The free space originally pointed to by
pool->next_non_full will be unavailable unless a subsequent sfree() call
frees allocated space above it. Resolve this issue by advancing
pool->next_non_full only outside the search loop and only when it points
to fully allocated space.