h264_cavlc: check prefix before using it.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_cavlc.c

@@ -513,7 +513,7 @@ static int decode_residual(H264Context *h, GetBitContext *gb, DCTELEM *block, in
                 else
                     level_code= prefix + get_bits(gb, 4); //part
             }else{
-                level_code= 30 + get_bits(gb, prefix-3); //part
+                level_code= 30;
                 if(prefix>=16){
                     if(prefix > 25+3){
                         av_log(h->s.avctx, AV_LOG_ERROR, "Invalid level prefix\n");
@@ -521,6 +521,7 @@ static int decode_residual(H264Context *h, GetBitContext *gb, DCTELEM *block, in
                     }
                     level_code += (1<<(prefix-3))-4096;
                 }
+                level_code += get_bits(gb, prefix-3); //part
             }
 
             if(trailing_ones < 3) level_code += 2;