[BUGFIX] Fix path check for custom RTE styles

Fix for #29899 introduced a check if the file holding
custom RTE CSS exists. However that path used was
wrong.

Change-Id: I4efb484a65d825dd6ce18bd8533f299c45782c67
Fixes: #32625
Releases: 4.5, 4.6, 4.7
Reviewed-on: http://review.typo3.org/7342
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland