Recurring Events Error

[jmbarne3]

There's a logical error in our clean function that's causing an Internal Server Error to be thrown when users save events under certain circumstances.

To reproduce:

1. Create a new event
2. Set the start date and end date fields, but leave the time field on both fields blank.
3. Change the recurrence settings to anything other than "Never" and set the Recur until field to a valid date.
4. Save the event after entering the rest of the content correctly.

The problem is without an end time set, the end variable will be none. While the error will get set properly that causes the error message on the front end, an exception occurs before the function returns because of this line. We need to ensure that the end variable is not None before trying to access any properties on it.