Merge branch 'tb/submodule-null-deref-fix' into jch

"git submodule" code trusted the data coming from the config (and
the in-tree .gitmodules file) too much without validating, leading
to NULL dereference if the user mucks with a repository (e.g.
submodule.<name>.url is removed).  This has been corrected.

* tb/submodule-null-deref-fix:
  builtin/submodule--helper.c: handle missing submodule URLs

builtin/submodule--helper.c

@@ -2027,14 +2027,17 @@ static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "submodule.%s.url", sub->name);
 	if (repo_config_get_string_tmp(the_repository, sb.buf, &url)) {
-		if (starts_with_dot_slash(sub->url) ||
-		    starts_with_dot_dot_slash(sub->url)) {
+		if (sub->url && (starts_with_dot_slash(sub->url) ||
+				 starts_with_dot_dot_slash(sub->url))) {
 			url = resolve_relative_url(sub->url, NULL, 0);
 			need_free_url = 1;
 		} else
 			url = sub->url;
 	}
 
+	if (!url)
+		die(_("cannot clone submodule '%s' without a URL"), sub->name);
+
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/.git", ce->name);
 	needs_cloning = !file_exists(sb.buf);

t/t7400-submodule-basic.sh

@@ -1351,6 +1351,22 @@ test_expect_success 'clone active submodule without submodule url set' '
 	)
 '
 
+test_expect_success 'update submodules without url set in .gitconfig' '
+	test_when_finished "rm -rf multisuper_clone" &&
+	git clone file://"$pwd"/multisuper multisuper_clone &&
+
+	git -C multisuper_clone submodule init &&
+	for s in sub0 sub1 sub2 sub3
+	do
+		key=submodule.$s.url &&
+		git -C multisuper_clone config --local --unset $key &&
+		git -C multisuper_clone config --file .gitmodules --unset $key || return 1
+	done &&
+
+	test_must_fail git -C multisuper_clone submodule update 2>err &&
+	grep "cannot clone submodule .sub[0-3]. without a URL" err
+'
+
 test_expect_success 'clone --recurse-submodules with a pathspec works' '
 	test_when_finished "rm -rf multisuper_clone" &&
 	cat >expected <<-\EOF &&