dxa: Make sure the reference frame exists

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit 5ef7c84a93)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Conflicts:
	libavcodec/dxa.c

libavcodec/dxa.c

@@ -256,6 +256,12 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame, AVPac
     case 5:
         c->pic.key_frame = !(compr & 1);
         c->pic.pict_type = (compr & 1) ? AV_PICTURE_TYPE_P : AV_PICTURE_TYPE_I;
+
+        if (!tmpptr && !c->pic.key_frame) {
+            av_log(avctx, AV_LOG_ERROR, "Missing reference frame.\n");
+            return AVERROR_INVALIDDATA;
+        }
+
         for(j = 0; j < avctx->height; j++){
             if(compr & 1){
                 for(i = 0; i < avctx->width; i++)