[BUGFIX] RTE: Uncaught error in Firefox in TYPO3 6.0

This is a regression introduced by fix to issue #41411. In Firefox,
the selection object may not provide any usable range.

Change-Id: I8f6e83ca249fb30299fa5b41755596ed54368343
Resolves: #41796
Releases: 6.0
Reviewed-on: http://review.typo3.org/15450
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland