Modify aset.c to track the next intended block allocation size explicitly.

Modify aset.c to track the next intended block allocation size explicitly.
The former coding relied on the actual allocated size of the last block,
which made it behave strangely if the first allocation in a context was
larger than ALLOC_CHUNK_LIMIT: subsequent allocations would be referenced
to that and not to the intended series of block sizes.  Noted while
studying a memory wastage gripe from Tatsuo.