[lipstick] Prevent lipstick from killing random apps. Fixes JB#29648
[lipstick] Prevent lipstick from killing random apps. Fixes JB#29648

Check that pid is positive before trying to kill.

With some bad luck, lcw can belong to an application that does no
longer have a surface, or does not have a client, or the surface
is in the process of being destroyed. In those cases, it is possible
that trying to terminate the process would call kill with
pid 0 or -1.

giucam/lipstick,sletta/lipstick,sletta/lipstick,giucam/lipstick,giucam/lipstick,sletta/lipstick