[znc] Recent system updates cause "stack smashing" segfault

[aphirst]

https://github.com/znc/znc/issues/1459

As covered, after recent system updates, ZNC runs for a time then suddenly segfaults, complaining of "stack smashing" attempts. This occurs with ZNC and with various -git versions, so the issue doesn't seem to be with ZNC itself.

The github user 0KiB managed to catch a list of package updates, after which the described issue is triggered. https://pastebin.com/7CdNc9Ny

[aphirst]

There have been some further investigative developments which I'll reproduce here.

https://github.com/znc/znc/issues/1459# ... -347027751

User "Bonstra" has determined that the culprit is in fact icu, "ZNC appears to crash while translating charsets for the result of a WHO command, for some users whose real name contains non-ASCII characters."

Compiling with --disable-charset obviously disables the character-set conversion functionality, but totally prevents the crashing.

User "psychon" believes the routine in question is "ucnv_convertEx", but isn't sure what the underlying cause is. I suspect it's worth seeing what changed in that routine, and the routines it calls, since the last icu release.