libpfctl: fix memory leak

When we create an nvlist and insert it into another nvlist we must
remember to destroy it. The nvlist_add_nvlist() function makes a copy,
just like nvlist_add_string() makes a copy of the string.

See also 4483fb47735c29408c72045469c9c4b3e549668b

Reviewed by: scottl
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30492