janitorial: pmts to ReadFile and WriteFile

[Eric Pouech]

for native compatibility, ReadFile and WriteFile cannot take at the same time a 
NULL pointer for the number of bytes read (resp. written) and a NULL overlapped 
structure.
This patch:
- spits this error in ReadFile & WriteFile
- fixes the places (I hope I didn't miss one) where we were using two NULL 
parameters

A+
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