core: adjust unit_get_ancestor_memory_{low,min}() to work with units which...

core: adjust unit_get_ancestor_memory_{low,min}() to work with units which don't have a CGroupContext

Coverity doesn't like the fact that unit_get_cgroup_context() returns NULL for
unit types that don't have a CGroupContext. We don't expect to call those
functions with such unit types, so this isn't an immediate problem, but we can
make things more robust by handling this case.

CID #1400683, #1400684.