Fix ReviewRequest.is_accessible_by for screenshot-only reviews
Fix ReviewRequest.is_accessible_by for screenshot-only reviews

This was calling into the repository field, which is None for screenshot-only
review requests. This is a trivial fix for that.

1tush/reviewboard,Khan/reviewboard,chazy/reviewboard,chazy/reviewboard,atagar/ReviewBoard,bkochendorfer/reviewboard,1tush/reviewboard,atagar/ReviewBoard,custode/reviewboard,atagar/ReviewBoard,1tush/reviewboard,reviewboard/reviewboard,atagar/ReviewBoard,1tush/reviewboard,chipx86/reviewboard,custode/reviewboard,Khan/reviewboard,chazy/reviewboard,brennie/reviewboard,reviewboard/reviewboard,1tush/reviewboard,Khan/reviewboard,chazy/reviewboard,1tush/reviewboard,KnowNo/reviewboard,chazy/reviewboard,Khan/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,davidt/reviewboard,1tush/reviewboard,beol/reviewboard,custode/reviewboard,brennie/reviewboard,atagar/ReviewBoard,brennie/reviewboard,atagar/ReviewBoard,KnowNo/reviewboard,chipx86/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,chazy/reviewboard,davidt/reviewboard,atagar/ReviewBoard,davidt/reviewboard,sgallagher/reviewboard,chazy/reviewboard,reviewboard/reviewboard,beol/reviewboard,reviewboard/reviewboard,bkochendorfer/reviewboard,chazy/reviewboard,sgallagher/reviewboard,atagar/ReviewBoard,KnowNo/reviewboard,Khan/reviewboard,KnowNo/reviewboard,Khan/reviewboard,Khan/reviewboard,chipx86/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,chazy/reviewboard,atagar/ReviewBoard,brennie/reviewboard,beol/reviewboard,sgallagher/reviewboard,Khan/reviewboard,Khan/reviewboard,custode/reviewboard,davidt/reviewboard,beol/reviewboard