ObjCFrameBuffer getBuffer callback can return nullptr

https://bugs.webkit.org/show_bug.cgi?id=259265
rdar://112220174

Reviewed by Eric Carlson.

RemoteVideoFrameProxy::pixelBuffer() is not guaranteed to always return a non null pixel buffer.
It is trying to do so by returning a black frame if grabbing the actual video frame fails.
But the black frame creation can itself fail.

To prevent this, we are updating ObjCFrameBuffer so that, should the pixel buffer be nullptr,
ObjCFrameBuffer::ToI420 will itself returns nullptr instead ofa I420BufferInterface which is empty inside.
The rest of libwebrtc deals with nullptr I420BufferInterface by exiting encoding early.

Manually tested by updating RemoteVideoFrameProxy::pixelBuffer() to return nullptr.

* Source/ThirdParty/libwebrtc/Source/webrtc/sdk/objc/native/src/objc_frame_buffer.mm:
(webrtc::ObjCFrameBuffer::ToI420):
(webrtc::ObjCFrameBuffer::wrapped_frame_buffer const):

Canonical link: https://commits.webkit.org/266097@main

Source/ThirdParty/libwebrtc/Source/webrtc/sdk/objc/native/src/objc_frame_buffer.mm

@@ -81,6 +81,9 @@ explicit ObjCI420FrameBuffer(id<RTCI420Buffer> frame_buffer)
 }
 
 rtc::scoped_refptr<I420BufferInterface> ObjCFrameBuffer::ToI420() {
+  auto frameBuffer = wrapped_frame_buffer();
+  if (!frameBuffer)
+    return nullptr;
   rtc::scoped_refptr<I420BufferInterface> buffer =
     rtc::make_ref_counted<ObjCI420FrameBuffer>([wrapped_frame_buffer() toI420]);
 
@@ -90,8 +93,10 @@ explicit ObjCI420FrameBuffer(id<RTCI420Buffer> frame_buffer)
 id<RTCVideoFrameBuffer> ObjCFrameBuffer::wrapped_frame_buffer() const {
 #if defined(WEBRTC_WEBKIT_BUILD)
   webrtc::MutexLock lock(&mutex_);
-  if (!frame_buffer_ && frame_buffer_provider_.getBuffer)
-    const_cast<ObjCFrameBuffer*>(this)->frame_buffer_ = [[RTCCVPixelBuffer alloc] initWithPixelBuffer:frame_buffer_provider_.getBuffer(frame_buffer_provider_.pointer)];
+  if (!frame_buffer_ && frame_buffer_provider_.getBuffer && frame_buffer_provider_.pointer) {
+    if (auto* buffer = frame_buffer_provider_.getBuffer(frame_buffer_provider_.pointer))
+      const_cast<ObjCFrameBuffer*>(this)->frame_buffer_ = [[RTCCVPixelBuffer alloc] initWithPixelBuffer:frame_buffer_provider_.getBuffer(frame_buffer_provider_.pointer)];
+  }
 #endif
   return frame_buffer_;
 }