lib-storage: Don't crash in mailbox_transaction_commit() if plugin aborts transaction.
lib-storage: Don't crash in mailbox_transaction_commit() if plugin aborts transaction.

jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch