fixed segmentation fault #758 (#762)

src/n2n.c

@@ -426,14 +426,15 @@ void resolve_cancel_thread (n2n_resolve_parameter_t *param) {
 uint8_t resolve_check (n2n_resolve_parameter_t *param, uint8_t requires_resolution, time_t now) {
 
     uint8_t ret = requires_resolution; /* if trylock fails, it still requires resolution */
+
 #ifdef HAVE_PTHREAD
+    if(param != NULL) {
         n2n_resolve_ip_sock_t   *entry, *tmp_entry;
         n2n_sock_str_t sock_buf;
 
         // check_interval and last_check do not need to be guarded by the mutex because
         // their values get changed and evaluated only here
 
-
         if((now - param->last_checked > param->check_interval) || (requires_resolution)) {
             // try to lock access
             if(pthread_mutex_trylock(&param->access) == 0) {
@@ -469,7 +470,9 @@ uint8_t resolve_check (n2n_resolve_parameter_t *param, uint8_t requires_resoluti
                 pthread_mutex_unlock(&param->access);
             }
         }
+    }
 #endif
+
     return ret;
 }