flicvideo: avoid an infinite loop in byte run compression

When byte_run is 0, pixel_countdown is not touched and the loop will run
forever.

CC:libav-stable@libav.org

libavcodec/flicvideo.c

@@ -348,6 +348,11 @@ static int flic_decode_frame_8BPP(AVCodecContext *avctx,
                 pixel_countdown = s->avctx->width;
                 while (pixel_countdown > 0) {
                     byte_run = sign_extend(bytestream2_get_byte(&g2), 8);
+                    if (!byte_run) {
+                        av_log(avctx, AV_LOG_ERROR, "Invalid byte run value.\n");
+                        return AVERROR_INVALIDDATA;
+                    }
+
                     if (byte_run > 0) {
                         palette_idx1 = bytestream2_get_byte(&g2);
                         CHECK_PIXEL_PTR(byte_run);