Bug 1472681: Fix NULL_DEREFERENCE error in js/src/vm/EnvironmentObject-inl.h. r=luke

MozReview-Commit-ID: 9p8jbWeZvhg

--HG--
extra : rebase_source : 9c163c7682be1e515dea2015019799ad69fa7d94

js/src/vm/EnvironmentObject-inl.h

@@ -17,8 +17,11 @@ namespace js {
 inline LexicalEnvironmentObject&
 NearestEnclosingExtensibleLexicalEnvironment(JSObject* env)
 {
-    while (!IsExtensibleLexicalEnvironment(env))
+    MOZ_ASSERT(env);
+    while (!IsExtensibleLexicalEnvironment(env)) {
         env = env->enclosingEnvironment();
+        MOZ_ASSERT(env);
+    }
     return env->as<LexicalEnvironmentObject>();
 }