libpfctl: fix memory leak

[kp]

Summary

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

MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")

[scottl]

This looks like a deficiency in the API design, but I don't object to this change.

[kp]

In D30492#685685, @scottl wrote:

This looks like a deficiency in the API design, but I don't object to this change.

You mean the nvlist API? Not really. This one is on me.
The API is consistent in copying data that's added to an nvlist with nvlist_add_*(). It does it with strings, and it also does it with nvlists. It does make sense, but I'd not thought of that when I originally wrote this code.