D7168: Handle processId being 0 in ShellClient::killWindow

[Martin Flöser]

graesslin added a project: FreeBSD.

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D7168

To: graesslin, #kwin, #plasma
Cc: plasma-devel, kwin, ZrenBot, progwolff, lesliezhai, ali-mohamed, hardening, 
jensreuterberg, abetts, sebas, apol, mart, lukas