update: job ID must be positive

The documentation for directory_update_init() was incorrect: a job ID
must be positive, not non-negative.  If the update queue is full and
no job was created, it makes more sense to return 0 instead of -1,
because it is more consistent with the return value of isUpdatingDB().

src/database.c

@@ -46,7 +46,7 @@ db_init(void)
 	music_root = directory_new("", NULL);
 
 	ret = directory_update_init(NULL);
-	if (ret < 0)
+	if (ret == 0)
 		FATAL("directory update failed\n");
 
 	do {