submodule: fix segmentation fault in submodule--helper clone

The git submodule--helper clone command will fail with a segmentation
fault when given a null url or null path variable. Since these are
required for proper functioning of the submodule--helper clone
subcommand, add checks to prevent running and fail gracefully when
missing.

Update the usage string to reflect the requirement that the --url and
--path "options" are required.

Signed-off-by: Jacob Keller <[email protected]>
Reviewed-by: Stefan Beller <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: jacob-keller

builtin/submodule--helper.c

@@ -180,8 +180,8 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 
 	const char *const git_submodule_helper_usage[] = {
 		N_("git submodule--helper clone [--prefix=<path>] [--quiet] "
-		   "[--reference <repository>] [--name <name>] [--url <url>]"
-		   "[--depth <depth>] [--path <path>]"),
+		   "[--reference <repository>] [--name <name>] [--depth <depth>] "
+		   "--url <url> --path <path>"),
 		NULL
 	};
 
@@ -191,7 +191,7 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 	if (!path || !*path)
 		die(_("submodule--helper: unspecified or empty --path"));
 
-	if (argc)
+	if (argc || !url)
 		usage_with_options(git_submodule_helper_usage,
 				   module_clone_options);