[BUGFIX] dont set fePreview=2 with ADMCMD_noBeUser

The workspace preview adds the GET parameter ADMCMD_noBeUser
when requesting the frame for live preview.

$GLOBALS[BE_USER] is unset in index_ts.php when seeing ADMCMD_noBeUser.

In TypoScriptFrontendController fePreview=2 must not be set when
ADMCMD_noBeUser parameter is present to prevent fatal errors on
calling methods on a non-existent $GLOBALS[BE_USER]

Releases: master, 6.2
Resolves: #30643
Change-Id: Idff7febcaaa8b87bd65a993b1860ad378e09b22f
Reviewed-on: http://review.typo3.org/27834


Reviewed-by: Lorenz Ulrich <lorenz.ulrich@visol.ch>
Tested-by: Lorenz Ulrich <lorenz.ulrich@visol.ch>
Reviewed-by: Markus Bucher <markusbucher@gmx.de>
Tested-by: Markus Bucher <markusbucher@gmx.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>