mpeg12enc: Don't set up run-level info for level 0.

run: The number of zero coefficients preceding a non-zero coefficient,
in the scan order. The absolute value of the non-zero coefficient is
called "level".

The run-level code makes illegal reads when trying to set up tables for
nonsense level 0.

libavcodec/mpeg12enc.c

@@ -74,6 +74,8 @@ static void init_uni_ac_vlc(RLTable *rl, uint8_t *uni_ac_vlc_len){
     for(i=0; i<128; i++){
         int level= i-64;
         int run;
+        if (!level)
+            continue;
         for(run=0; run<64; run++){
             int len, bits, code;