core/unit: fix superficial NULL-pointer dereference

core/unit: fix superficial NULL-pointer dereference

Though, when unit_prepare_exec() is called, the unit should always
have the cgroup runtime context 'crt'. So, I think we can insert assert().
But, for consistency with other places that call unit_get_cgroup_runtime(),
here use the simple non-NULL check for 'crt' instead of using assert().

Follow-up for 9cc545447e31ed38d71c00d5957c4dee0b5bce97.

Fixes CID#1534667.