hash: reduce memory pressure in hash_rehash no-op case

hash: reduce memory pressure in hash_rehash no-op case

* lib/hash.c (next_prime): Avoid overflow.
(hash_initialize): Factor bucket size computation...
(compute_bucket_size): ...into new helper function.
(hash_rehash): Use new function and open coding to reduce memory
pressure, and avoid a memory leak in USE_OBSTACK code.
Reported by Jim Meyering.

Signed-off-by: Eric Blake <ebb9@byu.net>