mpegvideo: unref old current_picture before allocating a new one

Allows decoding with only three frame buffers with frame reference
counting. Fixes VAAPI based decoding in vlc which uses only three
frame buffers for the mpegvideo-based codecs.

libavcodec/mpegvideo.c

@@ -1495,6 +1495,8 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
         }
     }
 
+    ff_mpeg_unref_picture(s, &s->current_picture);
+
     if (!s->encoding) {
         ff_release_unused_pictures(s, 1);
 
@@ -1542,7 +1544,6 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
     //     s->current_picture_ptr->quality = s->new_picture_ptr->quality;
     s->current_picture_ptr->f.key_frame = s->pict_type == AV_PICTURE_TYPE_I;
 
-    ff_mpeg_unref_picture(s, &s->current_picture);
     if ((ret = ff_mpeg_ref_picture(s, &s->current_picture,
                                    s->current_picture_ptr)) < 0)
         return ret;