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