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The value and holder iterator code had a past-the-end iterator
dereference.  While of course invalid, the dereference didn't actually
cause any problems (which is why it wasn't caught before) because the
dereferenced value is never actually used and `vector` implementations
appear to allow dereferencing the past-the-end iterator.  Under a MSVC
debug build, however, it fails a debug assertion and aborts.

This amends the iterator to just store and use a pointer to the vector
(rather than adding a second past-the-end iterator member), checking the
type index against the type vector size.