screen_browser: check for error before adding song to playlist

The function enqueue_and_play() may cause an assertion failure,
because it evaluates the filelist_entry object after
mpdclient_get_connection() call.  However, mpdclient_get_connection() may
indirectly invalidate the filelist_entry object, when it sees that the
connection has become invalid; it will return a NULL connection then.
This results in an assertion failure or a segmentation fault.