fix journalctl regression (#15099)

This regression was introduced in #14913.

The current_file variable can be NULL, as, for example, with the
following commands:

* journalctl --list-boots
* journalctl -b -1 --no-pager

Since current_file is only checked for pointer equality with f, removing
the assertion is safe here.

(cherry picked from commit 8d0726fc)