journal-gatewayd: fix segfault with certain request (#3893)

When client requests to get logs with `follow` and `KEY=match` that
doesn't match any log entry, journal-gatewayd segfaulted.

Make request_reader_entries to return zero in such case to wait for
matching entries.

This fixes https://github.com/systemd/systemd/issues/3873.

(cherry picked from commit 3475fc58)