MINOR: ncbuf: adjust ncb_data with NCBUF_NULL

Replace ncb_blk_is_null() by ncb_is_null() as a prelude to ncb_data().
The result is the same : the function will return 0 if the buffer is
uninitialized. However, it is clearer to directly call ncb_is_null() to
reflect this.

There is no functional change with this commit.
1 file changed