[status] fix getCount function

frackit/sampling/status.hh

@@ -147,8 +147,11 @@ public:
     /*!
      * \brief Returns the entity count for the given id.
      */
-    std::size_t getCount(const Id& id)
-    { return count_[id.get()]; }
+    std::size_t getCount(const Id& id) const
+    {
+        const auto it = count_.find(id.get());
+        return it != count_.end() ? it->second : 0;
+    }
 
     /*!
      * \brief Returns the overall entity count.