Dump current klass_ on bad ChangeClass

Change-Id: Idbe55d7e00a4b40993e6382b2bcfbb34ac39cc41
diff --git a/src/logging.cc b/src/logging.cc
index f962e41..4aacd3b 100644
--- a/src/logging.cc
+++ b/src/logging.cc
@@ -74,6 +74,11 @@
     return;
   }
 
+  if (address_ == NULL) {
+    os << "00000000:";
+    return;
+  }
+
   static const char gHexDigit[] = "0123456789abcdef";
   const unsigned char* addr = reinterpret_cast<const unsigned char*>(address_);
   char out[76];           /* exact fit */