fix for #496 (#497)

fix for the following warning: comparison between pointer and zero character constant (#496),
and try to solve login problems with certificate only (#375)

src/http.c

@@ -573,12 +573,13 @@ int auth_log_in(struct tunnel *tunnel)
 
 	tunnel->cookie[0] = '\0';
 
-	if (tunnel->config->use_engine) {
+	if (tunnel->config->use_engine
+	    || (username[0] == '\0' && tunnel->config->password[0] == '\0')) {
 		snprintf(data, sizeof(data), "cert=&nup=1");
 		ret = http_request(tunnel, "GET", "/remote/login",
 		                   data, &res, &response_size);
 	} else {
-		if (tunnel->config->password == '\0') {
+		if (tunnel->config->password[0] == '\0') {
 			snprintf(data, sizeof(data), "username=%s&realm=%s&ajax=1"
 			         "&redir=%%2Fremote%%2Findex&just_logged_in=1",
 			         username, realm);

src/main.c

@@ -512,8 +512,8 @@ int main(int argc, char **argv)
 		goto user_error;
 	}
 	// Check username
-	if (cfg.username[0] == '\0' && cfg.use_engine != 1) {
-		log_error("Specify an username.\n");
+	if (cfg.username[0] == '\0' && cfg.user_cert == NULL) {
+		log_error("Specify a username.\n");
 		goto user_error;
 	}
 	// If username but no password given, interactively ask user