error flushing io cache 

The reason for the error was incorrect return code from
my_win_write() in case of error on 64 bit Windows.

Error should be  indicated by return code 
(size_t)-1 == 2^64 -1, but due to cast it was
(DWORD)-1 = 2^32 -1

The caller of this function would fail to recognize the error
and continue looping.

Fix is to return correct error code (size_t)-1 in case of error
as expected by caller.

Also minimal cleanup is done : my_win_write() now uses
the same parameter checks  as related functions (0 and
overflow handling for count parameter).