ctladm: fix resource leak

The str variable in cctl_nvlist_end_element() does not get free()'d when
converted to an integer value. (name is "trtype")

Reported by: Coverity Scan
Coverity ID: 1545039
Sponsored by: The FreeBSD Foundation

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1237