Fix destructor for delayed mmap dataset case

[mikolajblaz]

What does this PR do ?

When delaying dataset mmap, dataset destructor errored out if the mmap creation was not actually called, leading to misleading tracebacks.

Collection: NLP

Changelog

• Fix MMapIndexedDataset destructor for delayed case.

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[mikolajblaz]

@ericharper can we merge this PR?

[ericharper]

LGTM. Thanks!