hash: factor, and guard against misbehaving hasher function

hash: factor, and guard against misbehaving hasher function

* lib/hash.c (safe_hasher): New function, to encapsulate the checking
of table->hasher's return value.  Also protect against a hash value
so large that adding it to table->bucket results in a NULL pointer.
(hash_lookup, hash_get_next, hash_find_entry, transfer_entries):
Use it in place of open-coded check-and-abort.