Ensure lastPixel.y is also initalized to -1 when necessary

Task-number: QTBUG-31579

Upstream-Status: Accepted [https://codereview.qt-project.org/#change,67578]
Signed-off-by: Jonathan Liu <net147@gmail.com>

Change-Id: Ia4916ac0d384ea523bbfad0dafbc90044da7ec3e
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>