cipher: Use offsetof for allocation of gcry_md_context.
* cipher/md.c (md_open): Allocate exact space for the allocation and use offsetof for the calculation.
Also, use hd->buf to get the pointer for CTX, so that compiler can
detect it's flexible array (for GCC 11).
- GnuPG-bug-id: T5556
- Signed-off-by: NIIBE Yutaka <email@example.com>