net-p2p/libtorrent: Fix remote DoS

Calls into build_benocde that use %zu could crash on 64 bit machines
due to the size change of size_t.
Someone can force READ_ENC_IA to fail allowing an internal_error to
be thrown and bring down the client, throw handshake_error instead.

PR: 224664
Submitted by: Henry David Bartholomew <PopularMoment@protonmail.com>
Approved by: maintainer timeout (pipfstarrd@openmailbox.org, > 2 weeks)
MFH: 2018Q1
Security: e4dd787e-0ea9-11e8-95f2-005056925db4