Prevent NULL dereference when the palette is missing in the xan decoder.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 56ee5a9ad1)

libavcodec/xan.c

@@ -552,6 +552,9 @@ static int xan_decode_frame(AVCodecContext *avctx,
         }
         buf_size = buf_end - buf;
     }
+    if (s->palettes_count <= 0)
+        return AVERROR_INVALIDDATA;
+
     if ((ret = avctx->get_buffer(avctx, &s->current_frame))) {
         av_log(s->avctx, AV_LOG_ERROR, "get_buffer() failed\n");
         return ret;