Fix macos

We don't have a resolver on macos, so we were running through this loop
with fails == 0 == m_Impls.size() and throwing, crashing the process.

Early return to avoid the failure and fix macos crash.

llarp/dns/platform.cpp

@@ -11,6 +11,8 @@ namespace llarp::dns
   void
   Multi_Platform::set_resolver(unsigned int index, llarp::SockAddr dns, bool global)
   {
+    if (m_Impls.empty())
+        return;
     size_t fails{0};
     for (const auto& ptr : m_Impls)
     {