Canvas: don't crash when getContext("2d") is called on destruction.

Canvas: don't crash when getContext("2d") is called on destruction.

We'd assume that a Window existed when that's not always the case.
For example:

import QtQuick 2.4
import QtQuick.Window 2.2
Window {
    Loader {
        sourceComponent: Canvas {
            Component.onDestruction: getContext("2d")
        }
        Component.onCompleted: active = false
    }
}

Change-Id: I5f30876e21ced5658adfa3bac494fd4196e0a7e3
Task-number: QTBUG-42222
Reviewed-by: Ulf Hermann <ulf.hermann@theqtcompany.com>