When we have a command returned by zfs_nextboot() that is longer
than command in the loader.conf, the latter needs to be nul terminated,
otherwise garbage trailer left from zfs_nextboot() will be passed to
parse_cmd() together with loader.conf command.

While here, reset cmd to empty string if read() returns error.

(cherry picked from commit c7dd069c70d465298f14553d981bfe5e2ffd0459)