Editorial: Add an assertion for privateEnv (#3485)

spec.html

@@ -20404,6 +20404,7 @@ <h1>Runtime Semantics: Evaluation</h1>
         1. Let _rVal_ be ? GetValue(_rRef_).
         1. If _rVal_ is not an Object, throw a *TypeError* exception.
         1. Let _privateEnv_ be the running execution context's PrivateEnvironment.
+        1. Assert: _privateEnv_ is not *null*.
         1. Let _privateName_ be ResolvePrivateIdentifier(_privateEnv_, _privateIdentifier_).
         1. If PrivateElementFind(_rVal_, _privateName_) is not ~empty~, return *true*.
         1. Return *false*.