mpegvideo: set reference/pict_type on generated reference frames

Otherwise the generic code will unref them, which can then result in
last_picture_ptr == current_picture_ptr, which causes deadlocks at least
in rv40.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/mpegvideo.c

@@ -1540,6 +1540,10 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             return i;
         }
         s->last_picture_ptr = &s->picture[i];
+
+        s->last_picture_ptr->reference   = 3;
+        s->last_picture_ptr->f.pict_type = AV_PICTURE_TYPE_I;
+
         if (ff_alloc_picture(s, s->last_picture_ptr, 0) < 0) {
             s->last_picture_ptr = NULL;
             return -1;
@@ -1567,6 +1571,10 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             return i;
         }
         s->next_picture_ptr = &s->picture[i];
+
+        s->next_picture_ptr->reference   = 3;
+        s->next_picture_ptr->f.pict_type = AV_PICTURE_TYPE_I;
+
         if (ff_alloc_picture(s, s->next_picture_ptr, 0) < 0) {
             s->next_picture_ptr = NULL;
             return -1;