core: simplify unit_escape_setting()

core: simplify unit_escape_setting()

The function had a provision for NULL input, and would return NULL, but that
looks like an error and all callers pass in a non-NULL arg and report oom on
NULL. So assert that the input is non-NULL.

All callers specifed the output buffer, so we can simplify the logic to only
make an allocation if appropriate and change the return type to 'const *'.

No functional change.