Fix null pointer dereference in xstepper operator*

Author: Alex-PLACET

include/xtensor/core/xiterator.hpp

@@ -483,10 +483,13 @@ namespace xt
     template <class C>
     inline auto xstepper<C>::operator*() const -> reference
     {
-        if (m_it == nullptr)
+        if constexpr (std::is_pointer<subiterator_type>::value)
         {
-            static std::remove_reference_t<reference> sentinel{};
-            return sentinel;
+            if (m_it == nullptr)
+            {
+                static std::remove_reference_t<reference> sentinel{};
+                return sentinel;
+            }
         }
         return *m_it;
     }