avcodec/mjpegenc_huffman: Assert length in ff_mjpegenc_huffman_compute_bits()

This should help coverity see that the issues this leads to cannot occur
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpegenc_huffman.c

@@ -87,6 +87,8 @@ void ff_mjpegenc_huffman_compute_bits(PTable *prob_table, HuffTable *distincts,
 
     int min;
 
+    av_assert0(max_length > 0);
+
     to->nitems = 0;
     from->nitems = 0;
     to->item_idx[0] = 0;