vlc_shared_data_ptr: accept implicit ctor for null

The main constructor, accepting a pointer, is explicit:

    my_ptr_type *p = ...;
    MySharedPtr ptr = p;   /* invalid */
    MySharedPtr ptr{ p };  /* ok */

This prevents to mistakenly assign a pointer to a shared pointer.

However, assignment to nullptr should be acceptable:

    MySharedPtr ptr = nullptr;  /* should be ok */

include/vlc_cxx_helpers.hpp

@@ -171,6 +171,11 @@ public:
             HOLD(ptr);
     }
 
+    vlc_shared_data_ptr(std::nullptr_t)
+        : ptr(nullptr)
+    {
+    }
+
     vlc_shared_data_ptr(const vlc_shared_data_ptr &other)
         : vlc_shared_data_ptr(other.ptr) {}