Fix copy construction and assignment from a null pointer.

This fixes FS 864.

[[Imported from SVN: r6322]]

dune/common/shared_ptr.hh

@@ -214,14 +214,17 @@ namespace Dune
   template<class T>
   inline shared_ptr<T>::shared_ptr(const shared_ptr<T>& other) : rep_(other.rep_)
   {
-    ++(rep_->count_);
+    if (rep_)
+      ++(rep_->count_);
   }
 
   template<class T>
   inline shared_ptr<T>& shared_ptr<T>::operator=(const shared_ptr<T>& other)
   {
-    (other.rep_->count_)++;
-    if(rep_!=0 && --(rep_->count_)<=0) delete rep_;
+    if (other.rep_) {
+      (other.rep_->count_)++;
+      if(rep_!=0 && --(rep_->count_)<=0) delete rep_;
+    }
     rep_ = other.rep_;
     return *this;
   }