fsck: do not abort upon finding an empty blob

fsck: do not abort upon finding an empty blob

Asking fwrite() to write one item of size bytes results in fwrite()
reporting "I wrote zero item", when size is zero. Instead, we could
ask it to write "size" items of 1 byte and expect it to report that
"I wrote size items" when it succeeds, with any value of size,
including zero.

Noticed and reported by BJ Hargrave.

Signed-off-by: Junio C Hamano <gitster@pobox.com>