[BUGFIX] Email links marked as external when preceded by external link

Variable is not initialized leading to incorrect transformation
on way to RTE.

Change-Id: I38751c6c5eb56920eb1286f2d39393b3dae7ce34
Resolves: #29782
Releases: 4.4, 4.5, 4.6
Reviewed-on: http://review.typo3.org/5537
Reviewed-by: Markus Klein
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch