Fix tests to pass with VerifyObject.

A SIRT ref with a null class was causing object verification problems
in space_test.

Change-Id: I95cfc3da5e0fec0155387f75612b0a3f26f6d3c9
diff --git a/runtime/indirect_reference_table.cc b/runtime/indirect_reference_table.cc
index ed3fb5f..bbad884 100644
--- a/runtime/indirect_reference_table.cc
+++ b/runtime/indirect_reference_table.cc
@@ -360,7 +360,9 @@
     return kInvalidIndirectRefObject;
   }
   mirror::Object* obj = table_[ExtractIndex(iref)];;
-  VerifyObject(obj);
+  if (obj != kClearedJniWeakGlobal) {
+    VerifyObject(obj);
+  }
   return obj;
 }