Guard against return value already being null

If there is no retrieved parameter, we should not de-reference null

Bug: 27748546
Change-Id: I8e6ceba26ab7d73ab365b72c0bfdcdb0a36a59a7

wpa_supplicant/config.c

@@ -2218,8 +2218,10 @@ char * wpa_config_get(struct wpa_ssid *ssid, const char *var)
 		const struct parse_data *field = &ssid_fields[i];
 		if (os_strcmp(var, field->name) == 0) {
 			char *ret = field->writer(field, ssid);
-			if (os_strchr(ret, '\r') != NULL || os_strchr(ret, '\n') != NULL) {
-				wpa_printf(MSG_ERROR, "Found newline in value for %s; "
+			if (ret != NULL && (os_strchr(ret, '\r') != NULL ||
+				os_strchr(ret, '\n') != NULL)) {
+				wpa_printf(MSG_ERROR,
+					"Found newline in value for %s; "
 					"not returning it", var);
 				os_free(ret);
 				ret = NULL;