Zap map value in the stub-cache on clear.

src/stub-cache.cc

Index: src/stub-cache.cc
diff --git a/src/stub-cache.cc b/src/stub-cache.cc
index a613ced0f609cce6469286eb84f38ab3f711ef09..4e3a906be906adfd4259b6ee1ed9bebe14ac5eaa 100644
--- a/src/stub-cache.cc
+++ b/src/stub-cache.cc
@@ -1034,10 +1034,12 @@ void StubCache::Clear() {
   Code* empty = isolate_->builtins()->builtin(Builtins::kIllegal);
   for (int i = 0; i < kPrimaryTableSize; i++) {
     primary_[i].key = heap()->empty_string();
+    primary_[i].map = NULL;
     primary_[i].value = empty;
   }
   for (int j = 0; j < kSecondaryTableSize; j++) {
     secondary_[j].key = heap()->empty_string();
+    secondary_[j].map = NULL;
     secondary_[j].value = empty;
   }
 }