sane-input-handler: fix power_then_voldown keybinding

hosts/common/programs/sane-input-handler/sane-input-handler

@@ -73,7 +73,7 @@ var Triggers = {
   PowerAndVolup    : "power_and_volup",
   PowerAndVoldown  : "power_and_voldown",
   PowerThenVolup   : "power_then_volup",
-  PowerThenVoldown : "power_and_voldown",
+  PowerThenVoldown : "power_then_voldown",
   VolupTap1        : "volup_tap_1",
   VolupTap2        : "volup_tap_2",
   VolupTap3        : "volup_tap_3",
@@ -457,6 +457,7 @@ func Dispatcher_add_handler(self, trigger, response; screen=false, fullscreen=fa
 func Dispatcher_new() {
   var handlers = {}
   for _k, v in (Triggers) {
+    assert (v not in handlers)
     setvar handlers[v] = []
   }
   return (Object(Dispatcher, { handlers: handlers }))