vp8: always update next_framep[] before returning from decode_frame().

Also slightly move around code not allocate a new frame if we won't
decode it. This prevents us from putting undecoded frames in frame
pointers, which (in mt decoding) other threads will use and wait on
as references, causing a deadlock (if we skipped decoding) or a crash
(if we didn't initialized next_framep[] at all).

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

libavcodec/vp8.c

@@ -1571,7 +1571,7 @@ static int vp8_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
     release_queued_segmaps(s, 0);
 
     if ((ret = decode_frame_header(s, avpkt->data, avpkt->size)) < 0)
-        return ret;
+        goto err;
 
     prev_frame = s->framep[VP56_FRAME_CURRENT];
 
@@ -1583,6 +1583,7 @@ static int vp8_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
 
     if (avctx->skip_frame >= skip_thresh) {
         s->invisible = 1;
+        memcpy(&s->next_framep[0], &s->framep[0], sizeof(s->framep[0]) * 4);
         goto skip_decode;
     }
     s->deblock_filter = s->filter.level && avctx->skip_loop_filter < skip_thresh;
@@ -1612,12 +1613,23 @@ static int vp8_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
     if (curframe->data[0])
         vp8_release_frame(s, curframe, 1, 0);
 
+    // Given that arithmetic probabilities are updated every frame, it's quite likely
+    // that the values we have on a random interframe are complete junk if we didn't
+    // start decode on a keyframe. So just don't display anything rather than junk.
+    if (!s->keyframe && (!s->framep[VP56_FRAME_PREVIOUS] ||
+                         !s->framep[VP56_FRAME_GOLDEN] ||
+                         !s->framep[VP56_FRAME_GOLDEN2])) {
+        av_log(avctx, AV_LOG_WARNING, "Discarding interframe without a prior keyframe!\n");
+        ret = AVERROR_INVALIDDATA;
+        goto err;
+    }
+
     curframe->key_frame = s->keyframe;
     curframe->pict_type = s->keyframe ? AV_PICTURE_TYPE_I : AV_PICTURE_TYPE_P;
     curframe->reference = referenced ? 3 : 0;
     if ((ret = vp8_alloc_frame(s, curframe))) {
         av_log(avctx, AV_LOG_ERROR, "get_buffer() failed!\n");
-        return ret;
+        goto err;
     }
 
     // check if golden and altref are swapped
@@ -1640,16 +1652,6 @@ static int vp8_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
 
     ff_thread_finish_setup(avctx);
 
-    // Given that arithmetic probabilities are updated every frame, it's quite likely
-    // that the values we have on a random interframe are complete junk if we didn't
-    // start decode on a keyframe. So just don't display anything rather than junk.
-    if (!s->keyframe && (!s->framep[VP56_FRAME_PREVIOUS] ||
-                         !s->framep[VP56_FRAME_GOLDEN] ||
-                         !s->framep[VP56_FRAME_GOLDEN2])) {
-        av_log(avctx, AV_LOG_WARNING, "Discarding interframe without a prior keyframe!\n");
-        return AVERROR_INVALIDDATA;
-    }
-
     s->linesize   = curframe->linesize[0];
     s->uvlinesize = curframe->linesize[1];
 
@@ -1759,20 +1761,23 @@ static int vp8_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
     }
 
     ff_thread_report_progress(curframe, INT_MAX, 0);
+    memcpy(&s->framep[0], &s->next_framep[0], sizeof(s->framep[0]) * 4);
+
 skip_decode:
     // if future frames don't use the updated probabilities,
     // reset them to the values we saved
     if (!s->update_probabilities)
         s->prob[0] = s->prob[1];
 
-    memcpy(&s->framep[0], &s->next_framep[0], sizeof(s->framep[0]) * 4);
-
     if (!s->invisible) {
         *(AVFrame*)data = *curframe;
         *data_size = sizeof(AVFrame);
     }
 
     return avpkt->size;
+err:
+    memcpy(&s->next_framep[0], &s->framep[0], sizeof(s->framep[0]) * 4);
+    return ret;
 }
 
 static av_cold int vp8_decode_init(AVCodecContext *avctx)