awful.screen.focus: Prevent focusing a screen that doesn't exist

Signed-off-by: Christopher Eatinger <christopher.eatinger@gmail.com>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/screen.lua.in

@@ -23,6 +23,7 @@ data.padding = {}
 --- Give the focus to a screen, and move pointer.
 -- @param screen Screen number.
 function focus(screen)
+    if screen > capi.screen.count() then screen = capi.mouse.screen end
     local c = client.focus.history.get(screen, 0)
     if c then capi.client.focus = c end
     -- Move the mouse on the screen