builtin/submodule--helper.c: handle missing submodule URLs

In e0a862fdaf (submodule helper: convert relative URL to absolute URL if needed, 2018-10-16), `prepare_to_clone_next_submodule()` lost the ability to handle URL-less submodules, due to a change from: if (repo_get_config_string_const(the_repostiory, sb.buf, &url)) url = sub->url; to if (repo_get_config_string_const(the_repostiory, sb.buf, &url)) { if (starts_with_dot_slash(sub->url) || starts_with_dot_dot_slash(sub->url)) { /* ... */ } } , which will segfault when `sub->url` is NULL, since both `starts_with_dot_slash()` does not guard its arguments as non-NULL. Guard the checks to both of the above functions by first checking whether `sub->url` is non-NULL. There is no need to check whether `sub` itself is NULL, since we already perform this check earlier in `prepare_to_clone_next_submodule()`. By adding a NULL-ness check on `sub->url`, we'll fall into the 'else' branch, setting `url` to `sub->url` (which is NULL). Before attempting to invoke `git submodule--helper clone`, check whether `url` is NULL, and die() if it is. Reported-by: Tribo Dar <3bodar@gmail.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>