mpeg12: skip frames consistently

The decoder did not start predicted frames with missing references but
called the end of frame handling on them anyway. This caused an
assertion in the VA API HW accelorator since it tried to render a
picture with uninitialized buffers.

libavcodec/mpeg12dec.c

@@ -2059,14 +2059,14 @@ static int decode_chunks(AVCodecContext *avctx,
     const uint8_t *buf_ptr = buf;
     const uint8_t *buf_end = buf + buf_size;
     int ret, input_size;
-    int last_code = 0;
+    int last_code = 0, skip_frame = 0;
 
     for (;;) {
         /* find next start code */
         uint32_t start_code = -1;
         buf_ptr = avpriv_find_start_code(buf_ptr, buf_end, &start_code);
         if (start_code > 0x1ff) {
-            if (s2->pict_type != AV_PICTURE_TYPE_B || avctx->skip_frame <= AVDISCARD_DEFAULT) {
+            if (!skip_frame) {
                 if (HAVE_THREADS && (avctx->active_thread_type & FF_THREAD_SLICE)) {
                     int i;
 
@@ -2210,20 +2210,27 @@ static int decode_chunks(AVCodecContext *avctx,
                 if (s2->last_picture_ptr == NULL) {
                 /* Skip B-frames if we do not have reference frames and gop is not closed */
                     if (s2->pict_type == AV_PICTURE_TYPE_B) {
-                        if (!s->closed_gop)
+                        if (!s->closed_gop) {
+                            skip_frame = 1;
                             break;
+                        }
                     }
                 }
                 if (s2->pict_type == AV_PICTURE_TYPE_I)
                     s->sync=1;
                 if (s2->next_picture_ptr == NULL) {
                 /* Skip P-frames if we do not have a reference frame or we have an invalid header. */
-                    if (s2->pict_type == AV_PICTURE_TYPE_P && !s->sync) break;
+                    if (s2->pict_type == AV_PICTURE_TYPE_P && !s->sync) {
+                        skip_frame = 1;
+                        break;
+                    }
                 }
                 if ((avctx->skip_frame >= AVDISCARD_NONREF && s2->pict_type == AV_PICTURE_TYPE_B) ||
                     (avctx->skip_frame >= AVDISCARD_NONKEY && s2->pict_type != AV_PICTURE_TYPE_I) ||
-                     avctx->skip_frame >= AVDISCARD_ALL)
+                     avctx->skip_frame >= AVDISCARD_ALL) {
+                    skip_frame = 1;
                     break;
+                }
 
                 if (!s->mpeg_enc_ctx_allocated)
                     break;
@@ -2241,6 +2248,7 @@ static int decode_chunks(AVCodecContext *avctx,
                 }
 
                 if (s2->first_slice) {
+                    skip_frame = 0;
                     s2->first_slice = 0;
                     if (mpeg_field_start(s2, buf, buf_size) < 0)
                         return -1;