And set *pid to -1.
[ChangeLog][QtCore][QProcess] If a startDetached() fails to start the
target application, the QProcess object should now have a proper error
string in errorString().
Pick-to: 6.1
Change-Id: Ic90d8429a0eb4837971dfffd1664e825ffcb923e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>