msvcrt: patch to set in _flsbuf the file->cnt to 0 if it gets negative and test case (#17123)

[Bernhard Übelacker]

---
 dlls/msvcrt/file.c       |    5 ++++-
 dlls/msvcrt/tests/file.c |   38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletions(-)
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