hash_object: correction for zero length file

hash_object: correction for zero length file

The check whether size is zero was done after if size <= SMALL_FILE_SIZE,
as result, zero size case was never triggered. Instead zero length file
was treated as any other small file. This did not caused any problem, but
if we have a special case for size equal to zero, it is better to make it
work and avoid redundant malloc().

Signed-off-by: Dmitry Potapov <redacted>
Signed-off-by: Junio C Hamano <redacted>