lib: fix _hash_member() crash w/ empty hash

lib: fix _hash_member() crash w/ empty hash

The typesafe hash _member() didn't check tabshift/count before
proceeding to look at the hash table, leading it to dereference a NULL
pointer when the hash table is in fact empty.

Test case added to tests/lib/test_typelist.

Note this function is not currently used anywhere.  Only lib/cspf.c uses
_member(), but it does so on a RB-tree rather than a hash.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>