Fix stupid error thrown on FF36 when trying to delete a nonexistent attribute from a DOM object.

common/content/events.js

@@ -992,7 +992,8 @@ var Events = Module("events", {
         */
 
         input: function onInput(event) {
-            delete event.originalTarget.dactylKeyPress;
+            if (event.originalTarget.dactylKeyPress)
+                delete event.originalTarget.dactylKeyPress;
         },
 
         // this keypress handler gets always called first, even if e.g.