libpfctl: fix Coverity issues

• handle snl_finalize_msg() returning NULL
• insert the correct data into the states list
• add missing nvlist_destroy()
• incorrect order for array bounds

Coverity: 1522929, 1522925, 1522923, 1522921, 1522780, 1522770, 1522764, 1487785, 1471250
Reviewed by: emaste
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42330