Avoid possible crash checking if vscr exists

This patch includes the check for vscr in handleClientMessage, because
the function wScreenForRootWindow could return NULL.

I check the calls to wScreenForRootWindow:

kix@osaka:~/src/awmaker/src$ grep wScreenForRootWindow *c
cycling.c:      virtual_screen  *vscr           = wScreenForRootWindow(event->xkey.root);
event.c:        vscr = wScreenForRootWindow(ev->xmaprequest.parent);
event.c:        virtual_screen *vscr = wScreenForRootWindow(event->xdestroywindow.event);
event.c:        vscr = wScreenForRootWindow(event->xbutton.root);
event.c:                vscr = wScreenForRootWindow(event->xclient.window);
event.c:                vscr = wScreenForRootWindow(event->xclient.window);
event.c:        virtual_screen *vscr = wScreenForRootWindow(event->xcrossing.root);
event.c:        virtual_screen *vscr = wScreenForRootWindow(event->xkey.root);
event.c:        virtual_screen *vscr = wScreenForRootWindow(event->xmotion.root);
rootmenu.c:     virtual_screen *vscr = wScreenForRootWindow(event->xkey.root);
startup.c:virtual_screen *wScreenForRootWindow(Window window)
startup.c:              return wScreenForRootWindow(attr.root);
kix@osaka:~/src/awmaker/src$

And the test is needed only in this function (at this moment).