[1.10>master] [MERGE #5391 @kfukuda2] Fixing null ScriptContext issue in JavascriptWeakMap

Merge pull request #5391 from kfukuda2:KeyGetLibraryBug

Fixes OS: 18010051
Fixing issue where ScriptContext can be null if closed before JavascriptWeakMap gets finalized

lib/Runtime/Library/JavascriptWeakMap.cpp

@@ -42,7 +42,7 @@ namespace Js
             return nullptr;
         }
 
-        if (key->GetScriptContext()->GetLibrary()->GetUndefined() == weakMapKeyData)
+        if (key->GetLibrary()->GetUndefined() == weakMapKeyData)
         {
             return nullptr;
         }