avformat/nutenc: Document trailer index assert better

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3a95b73a)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/nutenc.c

@@ -1172,7 +1172,7 @@ static int nut_write_trailer(AVFormatContext *s)
 
     ret = avio_open_dyn_buf(&dyn_bc);
     if (ret >= 0 && nut->sp_count) {
-        av_assert1(nut->write_index);
+        av_assert1(nut->write_index); // sp_count should be 0 if no index is going to be written
         write_index(nut, dyn_bc);
         put_packet(nut, bc, dyn_bc, 1, INDEX_STARTCODE);
     }