Permit insertRecurrenceItemAt to insert one past the end of the current list,

in which case it acts as appendRecurrenceItem.  (This means that
insertRecurrenceItemAt(0, item) always works, even if there have been no items
added already.)

calendar/base/src/calRecurrenceInfo.js

@@ -234,7 +234,7 @@ calRecurrenceInfo.prototype = {
         if (this.mImmutable)
             throw Components.results.NS_ERROR_OBJECT_IS_IMMUTABLE;
 
-        if (aIndex < 0 || aIndex >= this.mRecurrenceItems.length)
+        if (aIndex < 0 || aIndex > this.mRecurrenceItems.length)
             throw Components.results.NS_ERROR_INVALID_ARG;
 
         this.mRecurrenceItems.splice(aIndex, 0, aItem);