Resolve lint in VideoEncoder by setting avStream_ (#981)

Co-authored-by: Daniel Flores <[email protected]>

Author: Dan-Flores

src/torchcodec/_core/Encoder.h

@@ -162,7 +162,7 @@ class VideoEncoder {
 
   UniqueEncodingAVFormatContext avFormatContext_;
   UniqueAVCodecContext avCodecContext_;
-  AVStream* avStream_;
+  AVStream* avStream_ = nullptr;
   UniqueSwsContext swsContext_;
 
   const torch::Tensor frames_;