Fix segfault in preparser

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

src/playlist/preparser.c

@@ -170,13 +170,15 @@ static void Art( playlist_preparser_t *p_preparser, input_item_t *p_item )
         if( p_preparser->i_art_policy == ALBUM_ART_ALL &&
             ( !psz_arturl || strncmp( psz_arturl, "file://", 7 ) ) )
         {
-            msg_Dbg( p_playlist, "meta ok for %s, need to fetch art", psz_name );
+            msg_Dbg( p_playlist, "meta ok for %s, need to fetch art",
+                     psz_name ? psz_name : "(null)" );
             b_fetch = true;
         }
         else
         {
             msg_Dbg( p_playlist, "no fetch required for %s (art currently %s)",
-                     psz_name, psz_arturl );
+                     psz_name ? psz_name : "(null)",
+                     psz_arturl ? psz_arturl : "(null)" );
         }
     }
     vlc_mutex_unlock( &p_item->lock );