Avoid floating point exceptions in rasterizeLine.

Task-number: QTBUG-16757

Change-Id: I3ef988f1e039c12529d62197d85a62f201e4ae12
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
(cherry picked from qtbase/85b4f0d2d3ffd326548ca8f0e15ac27f5fd8d77d)
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>