Fix KMail crash when adding too many recipients

MultiplyingLineView::addLine() returns a null pointer if the limit
has been reached, but MultiplyingLineView::slotReturnPressed() uses
the pointer without a null check.

A quick crash fix. Still needs further investigation because the "too
many recipients" error box is shown when it should not be. See D28876
for more information.

Differential Revision: https://phabricator.kde.org/D28876