Fix and re-enable test-api/InitializeAndDispose.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index e0e90b375104758a68ee51848f719c8e62f50237..fdc09081681dbabc5a3a88d13810e9827123285f 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -99,10 +99,15 @@ void Bootstrapper::InitializeOncePerProcess() {
 
 void Bootstrapper::TearDownExtensions() {
   delete free_buffer_extension_;
+  free_buffer_extension_ = NULL;
   delete gc_extension_;
+  gc_extension_ = NULL;
   delete externalize_string_extension_;
+  externalize_string_extension_ = NULL;
   delete statistics_extension_;
+  statistics_extension_ = NULL;
   delete trigger_failure_extension_;
+  trigger_failure_extension_ = NULL;
 }