KSMTP: Fix sending emails where a line starts with a dot

Summary:
My previous fix was incomplete because it only considered lines
with a single dot, and not the case where two such lines followed each
other.

BUG: 392049
FIXED-IN: 18.04

Test Plan: Updated unittest.

Reviewers: dvratil, mlaurent

Subscribers: #kde_pim

Differential Revision: https://phabricator.kde.org/D12434