config.c: NULL check when reading protected config

In read_protected_config(), check whether each file name is NULL before
attempting to read it, and add a BUG() call to
git_config_from_file_with_options() to make this error easier to catch
in the future.

The NULL checks mirror what do_git_config_sequence() does (which
read_protected_config() is modeled after). Without these NULL checks,
multiple tests fail with "make SANITIZE=address", e.g. in the final test
of t4010, xdg_config is NULL causing us to call fopen(NULL).

Reported-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Glen Choo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: chooglen

config.c

@@ -1979,6 +1979,8 @@ int git_config_from_file_with_options(config_fn_t fn, const char *filename,
 	int ret = -1;
 	FILE *f;
 
+	if (!filename)
+		BUG("filename cannot be NULL");
 	f = fopen_or_warn(filename, "r");
 	if (f) {
 		ret = do_config_from_file(fn, CONFIG_ORIGIN_FILE, filename,
@@ -2645,9 +2647,12 @@ static void read_protected_config(void)
 	system_config = git_system_config();
 	git_global_config(&user_config, &xdg_config);
 
-	git_configset_add_file(&protected_config, system_config);
-	git_configset_add_file(&protected_config, xdg_config);
-	git_configset_add_file(&protected_config, user_config);
+	if (system_config)
+		git_configset_add_file(&protected_config, system_config);
+	if (xdg_config)
+		git_configset_add_file(&protected_config, xdg_config);
+	if (user_config)
+		git_configset_add_file(&protected_config, user_config);
 	git_configset_add_parameters(&protected_config);
 
 	free(system_config);