Crash during synchronization

[Yuri D'E. <wa...@th...>]

Using the sources on the current main branch:

| <snip>
| Logging in...
| Opening far side box INBOX...
| Opening near side box INBOX...
| Loading near side box...
| near side: 318 messages, 0 recent
| Loading far side box...
| far side: 462 messages, 0 recent
| Synchronizing...
| Socket error on <host> (<ip>:993): timeout.
| segmentation fault (core dumped)  mbsync -V -a

Timeout happening at the wrong moment... Stack trace is a bit limited,
since I foolishly built this without debug:

(gdb) where
#0  0x0000556531c46b99 in sync_boxes ()
#1  0x0000556531c4db78 in do_sync_boxes ()
#2  0x0000556531c3b240 in store_bad.lto_priv ()
#3  0x0000556531c2f305 in main_loop ()
#4  0x0000556531c4caa8 in main ()