[BUGFIX] Correct access of AJAX response in t3editor

Due to an update of prototype.js, the AJAX response has changed.
Use the correct response object to determine if saving was successful.

Resolves: #66380
Releases: master
Change-Id: I303d90e4cd80ee6164e26ad60d0ac3a1f5fe49c5
Reviewed-on: http://review.typo3.org/38635


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>