Fix clrhash bug when hash table needs rehashing

Fix clrhash bug when hash table needs rehashing

Problem reported by Pip Cet in:
https://lists.gnu.org/r/emacs-devel/2019-08/msg00316.html
* src/fns.c (maybe_resize_hash_table): Prefer ASET to gc_aset
where either will do.  Simplify appending of Qunbound values.
Put index_size calculation closer to where it’s needed.
(hash_clear): If hash_rehash_needed_p (h), don’t clear the
nonexistent hash vector.  Use memclear to speed up clearing.
* src/lisp.h (HASH_TABLE_SIZE): Check that the size is positive,
and tell that to the compiler.