config: don't segfault when given --path with a missing value

When given a variable without a value, such as '[section] var' and
asking git-config to treat it as a path, git_config_pathname returns
an error and doesn't modify its output parameter. show_config assumes
that the call is always successful and sets a variable to indicate
that vptr should be freed. In case of an error however, trying to do
this will cause the program to be killed, as it's pointing to memory
in the stack.

Detect the error and return immediately to avoid freeing or accessing
the uninitialed memory in the stack.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/config.c

@@ -129,7 +129,8 @@ static int show_config(const char *key_, const char *value_, void *cb)
 		else
 			sprintf(value, "%d", v);
 	} else if (types == TYPE_PATH) {
-		git_config_pathname(&vptr, key_, value_);
+		if (git_config_pathname(&vptr, key_, value_) < 0)
+			return -1;
 		must_free_vptr = 1;
 	} else if (value_) {
 		vptr = value_;

t/t1300-repo-config.sh

@@ -803,6 +803,11 @@ test_expect_success NOT_MINGW 'get --path copes with unset $HOME' '
 	test_cmp expect result
 '
 
+test_expect_success 'get --path barfs on boolean variable' '
+	echo "[path]bool" >.git/config &&
+	test_must_fail git config --get --path path.bool
+'
+
 cat > expect << EOF
 [quote]
 	leading = " test"