config --get --path: check for unset $HOME

If $HOME is unset (as in some automated build situations),
currently

	git config --path path.home "~"
	git config --path --get path.home

segfaults.  Error out with

	Failed to expand user dir in: '~/'

instead.

Reported-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -316,6 +316,8 @@ char *expand_user_path(const char *path)
 		size_t username_len = first_slash - username;
 		if (username_len == 0) {
 			const char *home = getenv("HOME");
+			if (!home)
+				goto return_null;
 			strbuf_add(&user_path, home, strlen(home));
 		} else {
 			struct passwd *pw = getpw_str(username, username_len);