Uploading a bad Diff in the UpdateDiff window causes a Start Over window.

When starting over, a valid diff creates a new review instead of updating.
Fixed by removing code that set the review request number to null.

The request number is now properly retained and no new review is created.

Testing done via reaching Start Over message, then uploading a proper diff. 

All results ended with the review being properly updated.
Tested invalid, unparsable and empty files to reach the "Start Over" page.

Tested (via modifying the code) to directly land on the "Start Over" page.

Tested uploading multiple incorrect files then a correct file.

Tested with multiple diff files.

Tested both by using the 'Select a diff' option and clicking and dragging.

Tested on both the Update page menu and the "New Review Request" page.

Tested via Jasmine that StartOver doesn't change reviewRequest.

All tests done on Firefox 48.0, Reviewboard 2.5.x and Python2.7.12