libnvmf: avoid resource leak

In nvmf_host_fetch_discovery_log_page(), the log variable may have been
allocated on the heap during the first loop cycle, and should be
free()'d before exiting upon errors.

Reported by: Coverity
CID: 1545034
Sponsored by: The FreeBSD Foundation

Reviewed by: imp,jhb
Pull Request: https://github.com/freebsd/freebsd-src/pull/1239