Clear out the Runtime singleton when destructing a Runtime instance.

Change-Id: Ib2c4699d997a33b52aeb3c63d54c325cf276faf7
diff --git a/src/runtime.cc b/src/runtime.cc
index eee84cd..f03157f 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -19,6 +19,9 @@
   delete class_linker_;
   Heap::Destroy();
   delete thread_list_;
+  // TODO: acquire a static mutex on Runtime to avoid racing.
+  CHECK(instance_ == this);
+  instance_ = NULL;
 }
 
 void Runtime::Abort(const char* file, int line) {