sha1-file: close fd of empty file in map_sha1_file_1()

sha1-file: close fd of empty file in map_sha1_file_1()

map_sha1_file_1() checks if the file it is about to mmap() is empty and
errors out in that case and explains the situation in an error message.
It leaks the private handle to that empty file, though.

Have the function clean up after itself and close the file descriptor
before exiting early.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>