nutenc: use av_assert

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/nutenc.c

@@ -810,7 +810,7 @@ static int nut_write_packet(AVFormatContext *s, AVPacket *pkt){
             frame_code=i;
         }
     }
-    assert(frame_code != -1);
+    av_assert0(frame_code != -1);
     fc= &nut->frame_code[frame_code];
     flags= fc->flags;
     needed_flags= get_needed_flags(nut, nus, fc, pkt);