[Bug 56837] Bad file pointer after a file write in appending mode

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=56837

--- Comment #13 from Piotr Caban  ---
The actual write to the file happens in _write function (not in fwrite). _write
calls _lseek before writing to the file. So we're only trying to fix value
returned by ftell/tell. The position where data is written to the file is not
changed by any of these patches.

It's possible to find where seek is happening. I have written some tests to
confirm it should not happen in every call to fwrite. I didn't add them to the
patch because I don't think it shows anything useful - I have added flsbuf()
tests instead.