journald: fix assertion failure on journal_file_link_data. (#5843)

When some error occurs during the initialization of JournalFile,
the JournalFile can be left without hash tables created.  When later
trying to append an entry to that file, the assertion in
journal_file_link_data() fails, and journald crashes.

This patch fix this issue by checking *_hash_table_size in
journal_file_verify_header().