segv in rbox_save_begin with Dovecot 2.2.21

[peter-mauritius]

During mail save the field dest_mail of the mail_save_context is NULL, which result in an segv. This happens at least with Dovecot 2.2.21.

Reproduced with IMAP Append.

[jrse]

struct mail_vfuncs->get_real_mail function handler changed, for dovecot 2.3 function pointer to index_mail_get_backend_mail is required. Currently used function handler index_mail_get_real_mail returns null.

[jrse]

mailbox_recent_flags_set_seqs is missing in dovecot-2.3 the function is used in rbox-sync to
mark the newly seen messages as recent