man:  use a variable to store the char * from node_get_url.

We get segfaults on some platforms when we do

    cmark_strbuf_printf(man, " (%s)", cmark_node_get_url(node));

but they go away with:

    url = cmark_node_get_url(node);
    cmark_strbuf_printf(man, " (%s)", url);

I don't understand why.

Closes #253.