message: Don't dereference null buffer in the LibaomAv1Encoder.
  No-Try: True
  Cr-Commit-Position: refs/heads/main@{#36377}
diff: @@ -631,8 +631,7 @@ int32_t LibaomAv1Encoder::Encode(
  (absl::c_find(supported_formats, mapped_buffer->type()) ==
  supported_formats.end() &&
  mapped_buffer->type() != VideoFrameBuffer::Type::kI420A)) {
  - rtc::scoped_refptr<I420BufferInterface> converted_buffer(
  - mapped_buffer->ToI420());
  + rtc::scoped_refptr<I420BufferInterface> converted_buffer(buffer->ToI420());
  if (!converted_buffer) {
  RTC_LOG(LS_ERROR) << "Failed to convert "
  << VideoFrameBufferTypeToString(
  