submodule: check return value of submodule_from_path()

As pointed out by CodeQL, it could be NULL and we usually check for
that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/submodule--helper.c

@@ -1907,6 +1907,9 @@ static int determine_submodule_update_strategy(struct repository *r,
 	const char *val;
 	int ret;
 
+	if (!sub)
+		return error(_("could not retrieve submodule information for path '%s'"), path);
+
 	key = xstrfmt("submodule.%s.update", sub->name);
 
 	if (update) {