wasm: Fix nullptr access in QWasmScreen::allwindowsHEADdev

If the QWindow::handle() has not been created yet we will add
a zero page pointer to the allwindows list.
One possible symptom is that emscripten::val complains
in context2d() that it is accessed by the wrong thread.
what happens is that the this pointer points to the first
page.
Seen in the documentviewer example

Change-Id: I2b08176ebdd7c35d5105194f7fd9f4f6d45b77e5
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

0 files changed, 0 insertions, 0 deletions