Fix xdr_bytes() strict-aliasing violations

Fix xdr_bytes() strict-aliasing violations

When xdr_bytes() is used for a gss_buffer_desc object, a temporary
character pointer must be used for the data value to avoid a strict
aliasing violation.

When xdr_bytes() is used for a krb5_keyblock object, a temporary
character pointer must also be used, even though the data pointer is
of type unsigned char *, to avoid a clang warning on macOS due to the
"#pragma pack" declaration in krb5.h.