Allow zero hash table size

Allow zero hash table size

This avoids any extra allocation for such vectors, including empty
tables read by the Lisp reader, and provides extra safety essentially
for free.

* src/fns.c (make_hash_table): Allow tables to be 0-sized.  The index
will always have at least one entry, to avoid extra look-up costs.
* src/alloc.c (process_mark_stack): Don't mark pure objects,
because empty vectors are pure.