avcodec/sunrast: Check for availability of maplength before allocating image

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 711ad71aea)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/sunrast.c

@@ -100,14 +100,14 @@ static int sunrast_decode_frame(AVCodecContext *avctx, void *data,
     if (ret < 0)
         return ret;
 
+    if (buf_end - buf < maplength)
+        return AVERROR_INVALIDDATA;
+
     if ((ret = ff_get_buffer(avctx, p, 0)) < 0)
         return ret;
 
     p->pict_type = AV_PICTURE_TYPE_I;
 
-    if (buf_end - buf < maplength)
-        return AVERROR_INVALIDDATA;
-
     if (depth > 8 && maplength) {
         av_log(avctx, AV_LOG_WARNING, "useless colormap found or file is corrupted, trying to recover\n");