[goobox] fixed crash

[Paolo Bacchilega <paobac src gnome org>]

commit 99946d1299910d6229d8757409709b39bc5bfcec
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Sun Dec 30 12:53:26 2018 +0100

    fixed crash

 src/goo-player-info.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/src/goo-player-info.c b/src/goo-player-info.c
index 8a7a64b..1a75e8d 100644
--- a/src/goo-player-info.c
+++ b/src/goo-player-info.c
@@ -533,8 +533,10 @@ goo_player_info_set_cover (GooPlayerInfo *info,
                return;
 
        g_clear_object (&info->priv->original_cover);
-       if (info->priv->cover_file != NULL)
+       if (info->priv->cover_file != NULL) {
                g_free (info->priv->cover_file);
+               info->priv->cover_file = NULL;
+       }
 
        if (strcmp (cover, "no-disc") == 0) {
                gtk_notebook_set_current_page (GTK_NOTEBOOK (info->priv->notebook), 0);