[MERGE #3835 @boingoing] Fix null dereference in LoadAllConstants whe…

…n code has no 'this' binding

Merge pull request #3835 from boingoing:FixEventHandlerThisBinding

Fixes https://microsoft.visualstudio.com/DefaultCollection/OS/_workitems?id=13977382&_a=edit

lib/Runtime/ByteCode/ByteCodeEmitter.cpp

@@ -1929,7 +1929,10 @@ void ByteCodeGenerator::LoadAllConstants(FuncInfo *funcInfo)
         byteCodeFunction->MapAndSetEnvRegister(funcInfo->GetEnvRegister());
         if (funcInfo->GetIsTopLevelEventHandler())
         {
-            byteCodeFunction->MapAndSetThisRegisterForEventHandler(funcInfo->GetThisSymbol()->GetLocation());
+            if (funcInfo->GetThisSymbol())
+            {
+                byteCodeFunction->MapAndSetThisRegisterForEventHandler(funcInfo->GetThisSymbol()->GetLocation());
+            }
             // The environment is the namespace hierarchy starting with "this".
             Assert(!funcInfo->RegIsConst(funcInfo->GetEnvRegister()));
             thisLoadedFromParams = true;