wincng.c: fixed possible memory leak in _libssh2_wincng_hash
wincng.c: fixed possible memory leak in _libssh2_wincng_hash

If _libssh2_wincng_hash_update failed _libssh2_wincng_hash_final
would never have been called before.

Reported by Zenju.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2