awful.client.setwfact: Handle invisible clients (FS#693)

setwfact() calls idx() on the client that it works on. When idx() is called with
a client that isn't visible, it fails while trying to find outs the client's
index in the current layout.

  event_handle_mousegrabber:119: error running function:
    lib/awful/client.lua:688: attempt to compare nil with number

Fix this by ignoring all clients that aren't visible in setwfact().

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/client.lua.in

@@ -724,8 +724,8 @@ end
 -- @param c the client
 function setwfact(wfact, c)
     -- get the currently selected window
-    local c =  c or capi.client.focus
-    if not c then return end
+    local c = c or capi.client.focus
+    if not c or not c:isvisible() then return end
 
     local t = tag.selected(c.screen)
     local w = idx(c)