BLI: add conversion constructor for destruct_ptr

This allows converting between different `destruct_ptr` types (which is
just a `std::unique_ptr` with a custom deleter).

The most common use case is to convert from a derived type to
the type of the base class.

source/blender/blenlib/BLI_memory_utils.hh

@@ -309,6 +309,12 @@ template<typename T> void uninitialized_fill_n(T *dst, int64_t n, const T &value
 }
 
 template<typename T> struct DestructValueAtAddress {
+  DestructValueAtAddress() = default;
+
+  template<typename U> DestructValueAtAddress(const U &)
+  {
+  }
+
   void operator()(T *ptr)
   {
     ptr->~T();