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.

src/journal/sd-journal.c

@@ -440,7 +440,6 @@ _pure_ static int compare_with_location(const JournalFile *f, const Location *l,
 
         assert(f);
         assert(l);
-        assert(current_file);
         assert(f->location_type == LOCATION_SEEK);
         assert(IN_SET(l->type, LOCATION_DISCRETE, LOCATION_SEEK));