Fix use of strncpy, which is actually good to use memcpy.

* common/ssh-utils.c (get_fingerprint): Use memcpy.
* g10/build-packet.c (string_to_notation): Use memcpy.

Cherry-pick of master commit of:
625ced6e672daa892d334323cce6b3d42a6f929f

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>