mss1: check number of free colours

Prevents out of array write.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/mss1.c

@@ -783,6 +783,10 @@ static av_cold int mss1_decode_init(AVCodecContext *avctx)
     av_log(avctx, AV_LOG_DEBUG, "Encoder version %d.%d\n",
            AV_RB32(avctx->extradata + 4), AV_RB32(avctx->extradata + 8));
     c->free_colours     = AV_RB32(avctx->extradata + 48);
+    if (c->free_colours < 0 || c->free_colours > 256) {
+        av_log(avctx, AV_LOG_ERROR, "Invalid free colours %d\n", c->free_colours);
+        return AVERROR_INVALIDDATA;
+    }
     av_log(avctx, AV_LOG_DEBUG, "%d free colour(s)\n", c->free_colours);
     avctx->coded_width  = AV_RB32(avctx->extradata + 20);
     avctx->coded_height = AV_RB32(avctx->extradata + 24);