Refactor: libcrmcommon: pcmk__parse_ll_range() accepts NULL output args

This allows it to be used as a syntax validator without needing dummy
output arguments. Currently nothing uses it in this way. The main
benefit here is that we can get rid of the cppcheck suppression.

Closes T881

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>