macro: upgrade ref counting overflow check assert() → assert_se()

macro: upgrade ref counting overflow check assert() → assert_se()

The overflow check for ref counting should not be subject to NDEBUG,
hence upgrade assert() → assert_se(). (The check for zero is an
immediate bug in our code, and should be impossible to trigger, hence
it's fine if the check is optimized away if people are crazy enough to
set NDEBUG, so that can stay assert())

https://github.com/systemd/systemd/pull/23099#discussion_r854341850