[eigen] fix broken `get_entry_ref`s

dune/xt/la/container/eigen/base.hh

@@ -162,12 +162,12 @@ public:
 protected:
   inline ScalarType& get_entry_ref(const size_t ii)
   {
-    return backend_[ii];
+    return (*backend_)[ii];
   }
 
   inline const ScalarType& get_entry_ref(const size_t ii) const
   {
-    return backend_[ii];
+    return (*backend_)[ii];
   }
 
 public: