cbs: Fragment/unit data is always reference counted

Make this clear in the documentation and add some asserts to ensure
that it is always true.

libavcodec/cbs.h

@@ -84,8 +84,9 @@ typedef struct CodedBitstreamUnit {
      */
     size_t   data_bit_padding;
     /**
-     * If data is reference counted, a reference to the buffer containing
-     * data.  Null if data is not reference counted.
+     * A reference to the buffer containing data.
+     *
+     * Must be set if data is not NULL.
      */
     AVBufferRef *data_ref;
 
@@ -130,8 +131,9 @@ typedef struct CodedBitstreamFragment {
      */
     size_t data_bit_padding;
     /**
-     * If data is reference counted, a reference to the buffer containing
-     * data.  Null if data is not reference counted.
+     * A reference to the buffer containing data.
+     *
+     * Must be set if data is not NULL.
      */
     AVBufferRef *data_ref;