pakfire_free: Avoid multiple calls

pakfire_free: Avoid multiple calls

If we want to access any objects that use reference counting, we will
increment and then decrement the reference counter of struct pakfire.
This will cause that pakfire_free() will be called multiple times which
will go wrong.

This change blocks that that will happen.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>