Update RTCVideoEncoderH264.mm (#6)

Author: hiroshihorie

sdk/objc/components/video_codec/RTCVideoEncoderH264.mm

@@ -572,6 +572,7 @@ - (BOOL)resetCompressionSessionIfNeededWithFrame:(RTC_OBJC_TYPE(RTCVideoFrame) *
   OSType framePixelFormat = [self pixelFormatOfFrame:frame];
 
   if (_compressionSession) {
+    _pixelBufferPool = VTCompressionSessionGetPixelBufferPool(_compressionSession);
     // The pool attribute `kCVPixelBufferPixelFormatTypeKey` can contain either an array of pixel
     // formats or a single pixel format.
     NSDictionary *poolAttributes =