<release 2.4>-<rc2> cherry-pick request: [tf.data] Fix snapshot segfault when using repeat and prefetch.

[yangustc07]

[tf.data] Fix snapshot segfault when using repeat and prefetch.

Fixes: #48903.

input_->MakeIterator refs the dataset in

tensorflow/tensorflow/core/framework/dataset.cc

Line 679 in a9cf3a0

params_.dataset->Ref();

So we don't need to call input_->Ref(). Otherwise, if
SnapshotDatasetV2Op::Dataset::Iterator::Reader::Initialize returns an error,
input_->Ref() isn't called, but the destructor still calls input_->Unref().

If InitializeIterator returns an error, the iterator_ needs to be reset to
nullptr. Otherwise, if GetNextInternal is called a second time,
iterator_->GetNext may dereference a null input_impl_.

@ashahab

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