avoid segfault on submodule.*.path set to an empty "true"

Git fails due to a segmentation fault if a submodule path is empty.
Here is an example .gitmodules that will cause a segmentation fault:

    [submodule "foo-module"]
      path
      url = http://host/repo.git
    $ git status
    Segmentation fault (core dumped)

This is because the parsing of "submodule.*.path" is not prepared to
see a value-less "true" and assumes that the value is always
non-NULL (parsing of "ignore" has the same problem).

Fix it by checking the NULL-ness of value and complain with
config_error_nonbool().

Signed-off-by: Jharrod LaFon <jlafon@eyesopen.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7400-submodule-basic.sh

@@ -18,6 +18,16 @@ test_expect_success 'setup - initial commit' '
 	git branch initial
 '
 
+test_expect_success 'configuration parsing' '
+	test_when_finished "rm -f .gitmodules" &&
+	cat >.gitmodules <<-\EOF &&
+	[submodule "s"]
+		path
+		ignore
+	EOF
+	test_must_fail git status
+'
+
 test_expect_success 'setup - repository in init subdirectory' '
 	mkdir init &&
 	(