entry: check for fstat() errors after checkout

entry: check for fstat() errors after checkout

In 11179eb311 ("entry.c: check if file exists after checkout",
2017-10-05) we started checking the result of the lstat() call done
after writing a file, to avoid writing garbage to the corresponding
cache entry. However, the code skips calling lstat() if it's possible
to use fstat() when it still has the file descriptor open. And when
calling fstat() we don't do the same error checking. To fix that, let
the callers of fstat_output() know when fstat() fails. In this case,
write_entry() will try to use lstat() and properly report an error if
that fails as well.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>