BUG: Check if the vscr exists in handleMapRequest

If the user select "Run..." in the rootmenu, the call to the function
wScreenForRootWindow returns NULL, so the next line:

WScreen *scr = vscr->screen_ptr

Make wmaker crash. This patch change the check:

"if (scr != NULL)"

to:

"if (vscr && vscr->screen_ptr)"

Therefore the vscr variable is checked too.