fix another integer overflow issue from switching to chrono

git-svn-id: http://svn.code.sf.net/p/libtorrent/code/trunk@10154 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix another integer overflow issue from switching to chrono

git-svn-id: http://svn.code.sf.net/p/libtorrent/code/trunk@10155 a83610d8-ad2a-0410-a6ab-fc0612d85776