[Users] ManageSieve looses filename

[Johan Vromans]

Claws-Mail 3.16.0-137-g5a69a4741 on Fedora 27.

When I [Edit] a sieve script using ManageSieve, a windows appears with the
sieve file contents and the sieve name in the title bar.

As soon as I actually change something, the sieve name in the title bar is
replaced by garbage. When I subsequently try to [Save] the sieve script, it
gives an error: Error in MANAGESIEVE command PUTSCRIPT: Invalid UTF-8
character in quoted-string. The script can no longer be saved.