[BUGFIX] MailUtility breakLinesForEmail cuts text wrong and discard rest

Now the substrStart pointer increases in case that the line is longer
than the lineWidth parameter.

Resolves: #49396
Releases: 6.2, 6.1, 6.0
Change-Id: I87b3622c2ada7efdcb4c1992c1cfa43635d68f09
Reviewed-on: https://review.typo3.org/21978
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind