Replace const-reference with pointer in SendData.

This argument is never used as a reference and the pointer that's bound
to the const reference may be nullptr. This is undefined behavior and
barks under UBSan.

BUG=webrtc:5124
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1642863003 .

Cr-Commit-Position: refs/heads/master@{#11418}

webrtc/modules/video_coding/include/video_coding_defines.h

@@ -59,7 +59,7 @@ class VCMPacketizationCallback {
  public:
   virtual int32_t SendData(uint8_t payloadType,
                            const EncodedImage& encoded_image,
-                           const RTPFragmentationHeader& fragmentationHeader,
+                           const RTPFragmentationHeader* fragmentationHeader,
                            const RTPVideoHeader* rtpVideoHdr) = 0;
 
   virtual void OnEncoderImplementationName(const char* implementation_name) {}