Fix iterator dereference for null pointer and improve strided_data_end checks

Author: Alex-PLACET

include/xtensor/core/xiterator.hpp

@@ -483,6 +483,11 @@ namespace xt
     template <class C>
     inline auto xstepper<C>::operator*() const -> reference
     {
+        if (m_it == nullptr)
+        {
+            static std::remove_reference_t<reference> sentinel{};
+            return sentinel;
+        }
         return *m_it;
     }
 

include/xtensor/core/xstrides.hpp

@@ -171,7 +171,11 @@ namespace xt
     It strided_data_end(const C& c, It begin, layout_type l, size_type offset)
     {
         using difference_type = typename std::iterator_traits<It>::difference_type;
-        if (c.dimension() == 0 || c.size() == 0)
+        if (c.size() == 0)
+        {
+            return begin;
+        }
+        if (c.dimension() == 0)
         {
             ++begin;
         }