parse_pack_header_option(): avoid unaligned memory writes

parse_pack_header_option(): avoid unaligned memory writes

In order to recreate a pack header in our in-memory buffer, we cast the
buffer to a "struct pack_header" and assign the individual fields. This
is reported to cause SIGBUS on sparc64 due to alignment issues.

We can work around this by using put_be32() which will write individual
bytes into the buffer.

Reported-by: Koakuma <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>