[BUGFIX][TESTS] Fix unit test cropIsMultibyteSafe

The unit test cropIsMultibyteSafe (class tslib_contentTest) fails, if
error reporting includes E_WARNING. In this case mb_strlen() throws an
error, as it is called with an empty string as second parameter
$charset. Initializing $GLOBALS['TSFE']->renderCharset in the setUp
method solves this.

Change-Id: If04278e3cbd01ad2f4b6b02c1b3df7cf89b251f0
Resolves: #26491
Releases: 4.6, 4.5
Reviewed-on: http://review.typo3.org/1785
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader