avcodec/qtrleenc: Use keyframe when no previous frame is available

If keeping a reference to an earlier frame failed, the next frame must
be an I frame for lack of reference frame. This commit implements this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit d5fc16a6a8)

libavcodec/qtrleenc.c

@@ -369,7 +369,8 @@ static int qtrle_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     if ((ret = ff_alloc_packet2(avctx, pkt, s->max_buf_size, 0)) < 0)
         return ret;
 
-    if (avctx->gop_size == 0 || (s->avctx->frame_number % avctx->gop_size) == 0) {
+    if (avctx->gop_size == 0 || !s->previous_frame->data[0] ||
+        (s->avctx->frame_number % avctx->gop_size) == 0) {
         /* I-Frame */
         s->key_frame = 1;
     } else {