another plasma5 session saving bug and fix

[Andriy Gapon]

https://bugs.kde.org/show_bug.cgi?id=491130
https://invent.kde.org/plasma/plasma-workspace/-/commit/8ee3defdf81e9b5ad3edef498d04fca0f4289e4a

It seems that the problem is that there is a binary "string" that can have 
embedded zeros but it is copied as a C-string resulting in a truncation.

I think that I've been affected by this problem and with the patch the things 
have been good so far.

-- 
Andriy Gapon