check Singleton::try_get for nullptr in `CoroDNSResolver::getDNSResol…

…verInstance`

Summary:
- during shutdown, an evb may still be running while the DNSModule singleton has been shutdown / destructed

- this fails any dns request with exception `::SHUTDOWN`

Reviewed By: jalopezsilva

Differential Revision: D51733861

fbshipit-source-id: 36eb2131924a6e2acd87b42f61900677519d1cd3

proxygen/lib/dns/DNSModule.cpp

@@ -27,7 +27,7 @@ namespace proxygen {
 static folly::Singleton<DNSModule> gDNSModule;
 
 std::shared_ptr<DNSModule> DNSModule::get() {
-  return folly::Singleton<DNSModule>::try_get();
+  return gDNSModule.try_get();
 }
 
 DNSModule::DNSModule() {