submodule-config: fix test binary crashing when no arguments given

Since arg[0] will be NULL without any argument here and starts_with()
does not like NULL-pointers.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-submodule-config.c

@@ -23,7 +23,7 @@ int main(int argc, char **argv)
 
 	arg++;
 	my_argc--;
-	while (starts_with(arg[0], "--")) {
+	while (arg[0] && starts_with(arg[0], "--")) {
 		if (!strcmp(arg[0], "--url"))
 			output_url = 1;
 		if (!strcmp(arg[0], "--name"))