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);

t/t1300-repo-config.sh

@@ -707,19 +707,41 @@ test_expect_success 'set --path' '
 	git config --path path.trailingtilde "foo~" &&
 	test_cmp expect .git/config'
 
+if test "${HOME+set}"
+then
+	test_set_prereq HOMEVAR
+fi
+
 cat >expect <<EOF
 $HOME/
 /dev/null
 foo~
 EOF
 
-test_expect_success 'get --path' '
+test_expect_success HOMEVAR 'get --path' '
 	git config --get --path path.home > result &&
 	git config --get --path path.normal >> result &&
 	git config --get --path path.trailingtilde >> result &&
 	test_cmp expect result
 '
 
+cat >expect <<\EOF
+/dev/null
+foo~
+EOF
+
+test_expect_success 'get --path copes with unset $HOME' '
+	(
+		unset HOME;
+		test_must_fail git config --get --path path.home \
+			>result 2>msg &&
+		git config --get --path path.normal >>result &&
+		git config --get --path path.trailingtilde >>result
+	) &&
+	grep "[Ff]ailed to expand.*~/" msg &&
+	test_cmp expect result
+'
+
 rm .git/config
 
 git config quote.leading " test"