expand_user_path: do not look at NULL path

We explicitly check for and handle the case that the
incoming "path" variable is NULL, but before doing so we
call strchrnul on it, leading to a potential segfault.

We can fix this simply by moving the strchrnul call down; as
a bonus, we can tighten the scope on the associated
variable.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -231,12 +231,12 @@ static struct passwd *getpw_str(const char *username, size_t len)
 char *expand_user_path(const char *path)
 {
 	struct strbuf user_path = STRBUF_INIT;
-	const char *first_slash = strchrnul(path, '/');
 	const char *to_copy = path;
 
 	if (path == NULL)
 		goto return_null;
 	if (path[0] == '~') {
+		const char *first_slash = strchrnul(path, '/');
 		const char *username = path + 1;
 		size_t username_len = first_slash - username;
 		if (username_len == 0) {