lib/awful/hooks: fix removal of last timer hook

Signed-off-by: Gregor Best <farhaven@googlemail.com>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/hooks.lua.in

@@ -112,6 +112,7 @@ for name, hook in pairs(capi.hooks) do
                 end
                 table.sort(delays)
                 _M[name].timer = delays[1]
+                if not delays[1] then delays[1] = 0 end
                 hook(delays[1], function (...)
                     for i, callback in ipairs(_M[name].callbacks) do
                         callback['counter'] = callback['counter'] + _M[name].timer