Check the relevant arguments for != NULL before calling the actual
driver-specific function, so that the driver can safely assume those
arguments are non-NULL. This removes the need to duplicate these
checks in every driver.
This patch might also work for a number of other Brymen models
-- 859(a), 867, 869---
including their respective rebadges from Greenlee, Extech, and Amprobe.
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>