libbacklight: Fix backlight never gets initialized
In 913d7c15 stricter error checking was introduced to the strtol call, which broke reading backlight values. Since every sysfs backlight file ends with a newline.
As noted in a comment in the previous MR to prevent damaged pointers after calling asprintf, replace every asprintf call with str_printf.
Previous-MR: !543 (closed)
Signed-off-by: Sören Meier soerenmeier@livgood.ch