[BUGFIX] RTE-API: correct check for backpath in writeTemporaryFile

\TYPO3\CMS\Rtehtmlarea\RteHtmlAreaApi::writeTemporaryFile needs to
also check regarding FE_MODE or frontend editing active like in
\TYPO3\CMS\Rtehtmlarea\RteHtmlAreaApi::getFullFileName to
resolve the backpath.

Otherwise extensions that create a TSFE object in the backend
but don't remove it, might result in wrong paths being used.

Change-Id: I7e16c4d7037d90c5dc044e5882455916e6c43a4b
Resolves: #71656
Releases: master
Reviewed-on: https://review.typo3.org/44769


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>

typo3/sysext/rtehtmlarea/Classes/RteHtmlAreaApi.php

@@ -320,7 +320,7 @@ abstract class RteHtmlAreaApi
                 throw new \RuntimeException($failure, 1294585668);
             }
         }
-        if (isset($GLOBALS['TSFE'])) {
+        if ($this->isFrontend() || $this->isFrontendEditActive()) {
             $fileName = $relativeFilename;
         } else {
             $fileName = '../' . $relativeFilename;