attr: make sure we have an xdg path before using it

If we don't have a core.attributesfile configured, we fall
back to checking XDG config, which is usually
$HOME/.config/git/attributes.

However, if $HOME is unset, then home_config_paths will return
NULL, and we end up calling fopen(NULL).

Depending on your system, this may or may not cause the
accompanying test to fail (e.g., on Linux and glibc, the
address will go straight to open, which will return EFAULT).
However, valgrind will reliably notice the error.

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

attr.c

@@ -520,11 +520,13 @@ static void bootstrap_attr_stack(void)
 		home_config_paths(NULL, &xdg_attributes_file, "attributes");
 		git_attributes_file = xdg_attributes_file;
 	}
-	elem = read_attr_from_file(git_attributes_file, 1);
-	if (elem) {
-		elem->origin = NULL;
-		elem->prev = attr_stack;
-		attr_stack = elem;
+	if (git_attributes_file) {
+		elem = read_attr_from_file(git_attributes_file, 1);
+		if (elem) {
+			elem->origin = NULL;
+			elem->prev = attr_stack;
+			attr_stack = elem;
+		}
 	}
 
 	if (!is_bare_repository() || direction == GIT_ATTR_INDEX) {

t/t1306-xdg-files.sh

@@ -106,6 +106,12 @@ test_expect_success 'Checking attributes in the XDG attributes file' '
 	test_cmp expected actual
 '
 
+test_expect_success 'Checking XDG attributes when HOME is unset' '
+	>expected &&
+	(sane_unset HOME &&
+	 git check-attr -a f >actual) &&
+	test_cmp expected actual
+'
 
 test_expect_success 'Checking attributes in both XDG and local attributes files' '
 	echo "f -attr_f" >.gitattributes &&