because backends manager frees that var already.
actually, adding ev_backends_manager_get_backends_dir to the public
API was still careless. backends manager and epub document jobs seem
to be in the different threads, and there's no mutex seen anywhere
around. luckily, what we have here is just a reading of a variable
that has been assigned only once (in backends_dir function), so it's
not as catastrophic as it would be if it was e.g. concurrent writing
without a mutex.
Closes https://github.com/mate-desktop/atril/pull/124