dds: disable palette flag for compressed dds

Having both is not valid and can cause a NULL pointer dereference of
frame->data[1] later.

Reviewed-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/dds.c

@@ -141,6 +141,12 @@ static int parse_pixel_format(AVCodecContext *avctx)
     normal_map      = flags & DDPF_NORMALMAP;
     fourcc = bytestream2_get_le32(gbc);
 
+    if (ctx->compressed && ctx->paletted) {
+        av_log(avctx, AV_LOG_WARNING,
+               "Disabling invalid palette flag for compressed dds.\n");
+        ctx->paletted = 0;
+    }
+
     bpp = bytestream2_get_le32(gbc); // rgbbitcount
     r   = bytestream2_get_le32(gbc); // rbitmask
     g   = bytestream2_get_le32(gbc); // gbitmask