[libfortran] Patch for pr 15328

[Bud Davis]

Hi ! 

It is great to have another person submitting patches.


In the routine finalize_transfer we have current_unit->s, which is of
type stream*, which gets changed to type unix_stream in routine flush..


My C is poor, but I think this is dangerous.  It works because the first
item in a unix_stream is a stream, but fd_flush will be updating items
in current_unit->s which do not exist, since it is not a unix_stream.


I think we need a way of mapping the a stream to a specific unix_stream.


Then again maybe I am just confused :)

Welcome aboard.

--bud