mac_veriexec_parser: Fix open_file error handling

NDFREE_PNBUF should be called after checking the return value of
vn_open(), and should only be called once.

Reviewed by: imp, zlei, Kornel Dulęba <mindal@semihalf.com>, Elliott Mitchell
Pull Request: https://github.com/freebsd/freebsd-src/pull/1338