smtp: enforce line limit even when LF is found

smtp: enforce line limit even when LF is found

Before:
If LF character was found, so far, we won't enforce the line limit on
the line. We only enforced limits in case of LF character missing in a
long line.

After this patch:
Line limit is enforced on the line if it is bigger than 4096 Bytes
irrespective of whether LF was found or not.

Redmine Bug: 5819

(cherry picked from commit 5f52b199ff0a49628fa00103d86bd72c5792a6b0)