Flush callback not called., causing the Transform stream to hang

[AhmedAlSaey]

Forgive me if my description for this issue is not clear enough. This is my first contribution and I am willing to spend time building a minimal reproduction environment for this issue, and fixing it in the future.

This issue happens when the following sequence of events occur:

1. Transformer _flush function is called first upon the read stream being transformed ends.
2. Passthrough stream this[kStream] ends using this[kStream].end. The end function's callback is not called yet.
3. fileType.fromStream promise resolves and the .then function is called, setting this[kStream] = null

After this sequence, which happens randomly but frequently, this callback function for _transform, which is called in the callback function of this[kStream].end will not be called, leaving the transformer hanging.

I think something about setting the passthrough stream to null prevents it's end callback function from being called. I am still not sure what the exact reason for this is, so if you have any insight, please let me know. Thanks.

[AhmedAlSaey]

This issue is a duplicate. Apologies for that. See #6