playlist: remove unnecessary shortcuts

Controls can be processed if the playlist is empty. They just do
nothing. Conversely, stop need not be processed if the playlist was
already killed.

src/playlist/control.c

@@ -53,8 +53,7 @@ static void playlist_vaControl( playlist_t *p_playlist, int i_query, va_list arg
 {
     PL_ASSERT_LOCKED;
 
-    if( i_query != PLAYLIST_STOP )
-        if( pl_priv(p_playlist)->killed || playlist_IsEmpty( p_playlist ) )
+    if( pl_priv(p_playlist)->killed )
         return;
 
     switch( i_query )