awful.widget.tasklist: Use pairs instead of ipairs

This is a weak table whose entries might be removed by the garbage collector.
I'm not sure if this will every cause problems, but it's better to be prepared
for tables having "nil holes" (which would mean ipairs ignores some entries).

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

lib/awful/widget/tasklist.lua

@@ -192,7 +192,7 @@ function tasklist.new(screen, filter, buttons, style, update_function, base_widg
             end
         end
         local function u()
-            for s in ipairs(instances) do
+            for s in pairs(instances) do
                 us(s)
             end
         end