core: assert that Ptr is not null on dereference

This way it is possible to catch errors even if the pointer is not actually dereferenced,
for example when it is used to call an object method or stored as a reference.

src/core/model/ptr.h

@@ -780,6 +780,7 @@ template <typename T>
 T *
 Ptr<T>::operator -> () 
 {
+  NS_ASSERT_MSG (m_ptr, "Attempted to dereference zero pointer");
   return m_ptr;
 }
 
@@ -787,6 +788,7 @@ template <typename T>
 T *
 Ptr<T>::operator -> () const
 {
+  NS_ASSERT_MSG (m_ptr, "Attempted to dereference zero pointer");
   return m_ptr;
 }
 
@@ -794,6 +796,7 @@ template <typename T>
 T &
 Ptr<T>::operator * () const
 {
+  NS_ASSERT_MSG (m_ptr, "Attempted to dereference zero pointer");
   return *m_ptr;
 }
 
@@ -801,6 +804,7 @@ template <typename T>
 T &
 Ptr<T>::operator * ()
 {
+  NS_ASSERT_MSG (m_ptr, "Attempted to dereference zero pointer");
   return *m_ptr;
 }