introduce sasprintf() in libi3, use it everywhere

src/manage.c

@@ -265,7 +265,7 @@ void manage_window(xcb_window_t window, xcb_get_window_attributes_cookie_t cooki
     nc->border_width = geom->border_width;
 
     char *name;
-    asprintf(&name, "[i3 con] container around %p", cwindow);
+    sasprintf(&name, "[i3 con] container around %p", cwindow);
     x_set_name(nc, name);
     free(name);