mpegvideo: Use the current_picture pts

The picture slot can be recycled by select_input_picture and
only current_picture is populated with the valid pts.

Unbreak timestamps when in cbr mode.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/mpegvideo_enc.c

@@ -1664,12 +1664,12 @@ vbv_retry:
         avctx->frame_bits  = s->frame_bits;
 
         pkt->pts = s->current_picture.f->pts;
-        if (!s->low_delay) {
+        if (!s->low_delay && s->pict_type != AV_PICTURE_TYPE_B) {
             if (!s->current_picture.f->coded_picture_number)
                 pkt->dts = pkt->pts - s->dts_delta;
             else
                 pkt->dts = s->reordered_pts;
-            s->reordered_pts = s->input_picture[0]->f->pts;
+            s->reordered_pts = pkt->pts;
         } else
             pkt->dts = pkt->pts;
         if (s->current_picture.f->key_frame)