Fix incwfact() error with maximized windows

When the window is maximized, calling incwfact() results in an error because `w` is nil, return instead.

lib/awful/client.lua

@@ -974,6 +974,8 @@ function client.incwfact(add, c)
 
     local t = c.screen.selected_tag
     local w = client.idx(c)
+    if not w then return end
+
     local data = t.windowfact or {}
     local colfact = data[w.col] or {}
     local curr = colfact[w.idx] or 1