player: reset frame step counter on seeks

I suppose this wasn't done in order to keep the frame step counter
active even in the next file, but actually it was reset anyway.

player/loadfile.c

@@ -903,7 +903,6 @@ static void play_current_file(struct MPContext *mpctx)
     mpctx->paused = false;
     mpctx->paused_for_cache = false;
     mpctx->playing_msg_shown = false;
-    mpctx->step_frames = 0;
     mpctx->backstep_active = false;
     mpctx->audio_delay = 0;
     mpctx->max_frames = -1;

player/playloop.c

@@ -154,6 +154,7 @@ void reset_playback_state(struct MPContext *mpctx)
     mpctx->playback_pts = MP_NOPTS_VALUE;
     mpctx->last_seek_pts = MP_NOPTS_VALUE;
     mpctx->cache_wait_time = 0;
+    mpctx->step_frames = 0;
     mpctx->restart_complete = false;
 
 #if HAVE_ENCODING