avcodec/dxv: Check idx in dxv_decompress_dxt5()

Fixes potential out of array read

No testcase available
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dxv.c

@@ -260,6 +260,10 @@ static int dxv_decompress_dxt5(AVCodecContext *avctx)
             case 2:
                 /* Copy two dwords from a previous index */
                 idx = 8 + bytestream2_get_le16(gbc);
+                if (idx > pos) {
+                    av_log(avctx, AV_LOG_ERROR, "idx %d > %d\n", idx, pos);
+                    return AVERROR_INVALIDDATA;
+                }
                 prev = AV_RL32(ctx->tex_data + 4 * (pos - idx));
                 AV_WL32(ctx->tex_data + 4 * pos, prev);
                 pos++;