Make a type conversion explicit. When compiling this code on
Windows as part of the SCTP userland stack, this fixes a
warning reported by Peter Kasting from Google.

MFC after: 3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271670 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f