delete LogDestination::sinks_ at DeleteLogDestinations().

fixes #8 AddLogSink memory leak
diff --git a/src/logging.cc b/src/logging.cc
index 5543e3a..091b0db 100644
--- a/src/logging.cc
+++ b/src/logging.cc
@@ -817,6 +817,11 @@
     delete log_destinations_[severity];
     log_destinations_[severity] = NULL;
   }
+  MutexLock l(&sink_mutex_);
+  for (size_t i = 0; i < sinks_->size(); ++i) {
+    delete (*sinks_)[i];
+  }
+  delete sinks_;
 }
 
 namespace {