[PATCH v3] 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>
---
Range-diff against v2:
1:  38202878b5 ! 1:  ae6cf3fa46 builtin/submodule--helper.c: handle missing submodule URLs
    @@ Commit message
         , 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 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
    +    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'
    @@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const st
      	strbuf_reset(&sb);
      	strbuf_addf(&sb, "%s/.git", ce->name);
      	needs_cloning = !file_exists(sb.buf);
    -@@ builtin/submodule--helper.c: static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
    - 	if (suc->update_data->filter_options && suc->update_data->filter_options->choice)
    - 		strvec_pushf(&child->args, "--filter=%s",
    - 			     expand_list_objects_filter_spec(suc->update_data->filter_options));
    -+	strvec_pushl(&child->args, "--url", url, NULL);
    - 	if (suc->update_data->require_init)
    - 		strvec_push(&child->args, "--require-init");
    - 	strvec_pushl(&child->args, "--path", sub->path, NULL);
    - 	strvec_pushl(&child->args, "--name", sub->name, NULL);
    --	strvec_pushl(&child->args, "--url", url, NULL);
    - 	if (suc->update_data->references.nr) {
    - 		struct string_list_item *item;
    - 
     
      ## t/t7400-submodule-basic.sh ##
     @@ t/t7400-submodule-basic.sh: test_expect_success 'clone active submodule without submodule url set' '

 builtin/submodule--helper.c |  7 +++++--
 t/t7400-submodule-basic.sh  | 16 ++++++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)