journald: fix another bad memory access

If we rotate due to header out of date we need the new journal file
object, too.

src/journal/journald.c

@@ -478,6 +478,10 @@ static void write_to_journal(Server *s, uid_t uid, struct iovec *iovec, unsigned
                 server_rotate(s);
                 server_vacuum(s);
                 vacuumed = true;
+
+                f = find_journal(s, uid);
+                if (!f)
+                        return;
         }
 
         for (;;) {