Bug 1615851 - layer::Image::GetAsSourceSurface is fallible. r=lsalzman

This fixes a crash, where we expected it to be infallible.

Differential Revision: https://phabricator.services.mozilla.com/D63005

--HG--
extra : moz-landing-system : lando

dom/canvas/ImageBitmap.cpp

@@ -615,6 +615,12 @@ UniquePtr<ImageBitmapCloneData> ImageBitmap::ToCloneData() const {
   result->mPictureRect = mPictureRect;
   result->mAlphaType = mAlphaType;
   RefPtr<SourceSurface> surface = mData->GetAsSourceSurface();
+  if (!surface) {
+    // It might just not be possible to get/map the surface. (e.g. from another
+    // process)
+    return nullptr;
+  }
+
   result->mSurface = surface->GetDataSurface();
   MOZ_ASSERT(result->mSurface);
   result->mWriteOnly = mWriteOnly;