Assert that the mark stack is empty when destroyed.

Change-Id: Ic1c2c9c02e487aad37bbc92ac28f1a6c5ad3ecbc
diff --git a/src/mark_stack.cc b/src/mark_stack.cc
index b61597b..707b98b 100644
--- a/src/mark_stack.cc
+++ b/src/mark_stack.cc
@@ -36,6 +36,8 @@
   }
 }
 
-MarkStack::~MarkStack() {}
+MarkStack::~MarkStack() {
+  CHECK(IsEmpty());
+}
 
 }  // namespace art