bug fix for wap when invert_hash is on

Hi John I believe I find what caused the bug,

--wap has its own routine for saving audited features, which is used in invert_hash. Since the hash_inv flag was not added to wap, the invert_hash routine may try to access to something like NULL.

I added the hash_inv flag beside the two audit flags in wap.cc, and it works now.