Merge pull request #123 from yying/master

Set sinks_ to NULL after deletion in LogDestination::DeleteLogDestinations
diff --git a/src/logging.cc b/src/logging.cc
index ec9eef1..0b5e6ee 100644
--- a/src/logging.cc
+++ b/src/logging.cc
@@ -825,6 +825,7 @@
   }
   MutexLock l(&sink_mutex_);
   delete sinks_;
+  sinks_ = NULL;
 }
 
 namespace {