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

@@ -515,7 +515,7 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 		usage_with_options(git_submodule_helper_usage,
 				   module_clone_options);
 
-	if (argc)
+	if (argc || !url)
 		usage_with_options(git_submodule_helper_usage,
 				   module_clone_options);