[Dovecot] 2.0.beta6 panic (mailbox_list_get_unexpanded_path)

[Mike Abbott]

2.0.beta6 assert-fails for me when:
- using mdbox (maildir is OK)
- my authentication module returns the mail= and mail_location= attributes (relying on mail_location from 10-mail.conf is OK)
- a user logs in for the first time and selects his nonexistent INBOX

The error is:
Panic: file mailbox-list.c: line 318 (mailbox_list_get_unexpanded_path): assertion failed: (*location == '0')

The location string is "1mdbox:/Volumes/Mail/joe".

This did not happen with beta5.

Backtrace:
0   __kill + 10
1   abort + 90
2   default_fatal_finish + 68
3   i_internal_error_handler + 0
4   i_debug + 0
5   mailbox_list_get_unexpanded_path + 111
6   mailbox_list_mkdir + 70
7   mdbox_map_open_internal + 169
8   mdbox_map_open_or_create + 14
9   mdbox_write_index_header + 51
10  mdbox_mailbox_create_indexes + 27
11  dbox_mailbox_create + 155
12  mailbox_open_full + 267
13  cmd_select_full + 1856 (cmd-select.c:287)
14  cmd_select + 11 (cmd-select.c:430)
15  client_command_input + 36 (imap-client.c:672)
16  client_command_input + 416 (imap-client.c:737)
17  client_handle_input + 134 (imap-client.c:778)
18  client_input + 118 (imap-client.c:829)
19  io_loop_handler_run + 234
20  io_loop_run + 56
21  master_service_run + 27
22  main + 689 (main.c:360)
23  start + 52