gh-62260: Fix ctypes.Structure subclassing with multiple layers (GH-13374)

The length field of StgDictObject for Structure class contains now
the total number of items in ffi_type_pointer.elements (excluding
the trailing null).

The old behavior of using the number of elements in the parent class can
cause the array to be truncated when it is copied, especially when there
are multiple layers of subclassing.

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>