Fix bob/eob checks

Fix bob/eob checks

* lisp/whitespace.el (whitespace-empty-at-bob-regexp)
(whitespace-empty-at-eob-regexp, whitespace-cleanup): Make `empty' checks
work at beginning & end of buffer in whitespace-report-region, and only
there (i.e. they will never match in regions that don't include the
start or end of the buffer).  (Bug#24745)