[BUGFIX] RTE: In IE, the pasting pad sometimes shows an error page

In IE, the pasting pad for plain text pasting sometimes shows an
error page.
Apparently, a timing issue.

Releases: master, 6.2
Resolves: #62658
Change-Id: Idb34ea6bc00d6020da637addee42cfd2db2d87f3
Reviewed-on: http://review.typo3.org/33758
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>