os: Fix NULL pointer dereference
RemoveHost() can be called from DisableLocalHost() with a NULL client, but doesn't actually check whether the given client pointer is valid on error and assigns the error value unconditionally, leading to a possible NULL pointer dereference and a crash of the Xserver.
To avoid the issue, simply check whether the client pointer is not NULL prior to assign the errorValue.
Closes: #1752 (closed)
See-also: https://bugzilla.redhat.com/2313799
Edited by Olivier Fourdan