Null deref under DynamicContentScalingImageBufferBackend::createBacke…

…ndHandle

https://bugs.webkit.org/show_bug.cgi?id=266723
rdar://119854411

Reviewed by Aditya Keerthi.

* Source/WebKit/Shared/RemoteLayerTree/DynamicContentScalingImageBufferBackend.mm:
(WebKit::DynamicContentScalingImageBufferBackend::createBackendHandle const):
Add a null check, since we have evidence that we can be asked to make a backend
handle when nobody has painted into the layer. I'm not sure why, but it's certainly
preferable to just not have a display list than to crash.

Canonical link: https://commits.webkit.org/272380@main

Source/WebKit/Shared/RemoteLayerTree/DynamicContentScalingImageBufferBackend.mm

@@ -91,7 +91,8 @@ static CFDictionaryRef makeContextOptions(const DynamicContentScalingImageBuffer
 
 std::optional<ImageBufferBackendHandle> DynamicContentScalingImageBufferBackend::createBackendHandle(SharedMemory::Protection) const
 {
-    ASSERT(m_context);
+    if (!m_context)
+        return std::nullopt;
 
     RetainPtr<NSDictionary> options;
     RetainPtr<NSMutableArray> ports;