Don't crash if QQuickPath has less than 4 elements

When running the Qt Cinematic demo on Android, the assertion
ASSERT: "i >= 0 && i < elementCount()" in painting/qpainterpath.cpp
failed because of a zero default size at startup.

Change-Id: I2c46c43319cf71030cf03b1f7c6cdaec782ce11a
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>