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: https://gitlab.freedesktop.org/xorg/xserver/-/issues/1752
See-also: https://bugzilla.redhat.com/2313799
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1701>

os/access.c

@@ -1413,12 +1413,14 @@ RemoveHost(ClientPtr client, int family, unsigned length,       /* of bytes in p
     case FamilyChaos:
     case FamilyServerInterpreted:
         if ((len = CheckAddr(family, pAddr, length)) < 0) {
+            if (client)
                 client->errorValue = length;
             return BadValue;
         }
         break;
     case FamilyLocal:
     default:
+        if (client)
             client->errorValue = family;
         return BadValue;
     }