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>

submodule.c

diff --git a/submodule.c b/submodule.c
index 975bc87..8f7515e 100644
--- a/submodule.c
+++ b/submodule.c
@@ -134,6 +134,9 @@
 		return 0;
 
 	if (!strcmp(key, "path")) {
+		if (!value)
+			return config_error_nonbool(var);
+
 		config = unsorted_string_list_lookup(&config_name_for_path, value);
 		if (config)
 			free(config->util);
@@ -151,6 +154,9 @@
 	} else if (!strcmp(key, "ignore")) {
 		char *name_cstr;
 
+		if (!value)
+			return config_error_nonbool(var);
+
 		if (strcmp(value, "untracked") && strcmp(value, "dirty") &&
 		    strcmp(value, "all") && strcmp(value, "none")) {
 			warning("Invalid parameter \"%s\" for config option \"submodule.%s.ignore\"", value, var);

t/t7400-submodule-basic.sh

diff --git a/t/t7400-submodule-basic.sh b/t/t7400-submodule-basic.sh
index 2683cba..0edf74c 100755
--- a/t/t7400-submodule-basic.sh
+++ b/t/t7400-submodule-basic.sh
@@ -18,6 +18,16 @@
 	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 &&
 	(