[BUGFIX] RTE: Parsing of some style sheets may fail in IE on first load

The length of the style sheet's rules array is reported as 0 while,
in fact, the style sheet is not yet completely loaded.

Releases: master
Resolves: #64854
Change-Id: I406ddebb250c15cd5b04ae1b734f146bd1025863
Reviewed-on: http://review.typo3.org/36656


Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>