hashmap: allow re-use after hashmap_free()

hashmap: allow re-use after hashmap_free()

Previously, once map->table had been freed, any calls to hashmap_put(),
hashmap_get(), or hashmap_remove() would cause a NULL pointer
dereference (since hashmap_free_() also zeros the memory; without that
zeroing, calling these functions would cause a use-after-free problem).

Modify these functions to check for a NULL table and automatically
allocate as needed.

Also add a HASHMAP_INIT(fn, data) macro for initializing hashmaps on the
stack without calling hashmap_init().

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>