player: added some more assertions

src/player_control.c

@@ -57,6 +57,8 @@ pc_song_deleted(const struct song *song)
 
 static void player_command(enum player_command cmd)
 {
+	assert(pc.command == PLAYER_COMMAND_NONE);
+
 	pc.command = cmd;
 	while (pc.command != PLAYER_COMMAND_NONE) {
 		notify_signal(&pc.notify);

src/player_thread.c

@@ -526,6 +526,8 @@ static gpointer player_task(G_GNUC_UNUSED gpointer arg)
 		switch (pc.command) {
 		case PLAYER_COMMAND_PLAY:
 		case PLAYER_COMMAND_QUEUE:
+			assert(pc.next_song != NULL);
+
 			do_play();
 			break;