Problem: build broken on kFreeBSD

Solution: fix refactor mistake introduced by:

091df74

Fixes #4113

Author: bluca

src/stream_engine_base.cpp

@@ -84,7 +84,7 @@ static std::string get_peer_address (zmq::fd_t s_)
     else if (family == PF_UNIX) {
         struct xucred cred;
         socklen_t size = sizeof (cred);
-        if (!getsockopt (_s, 0, LOCAL_PEERCRED, &cred, &size)
+        if (!getsockopt (s_, 0, LOCAL_PEERCRED, &cred, &size)
             && cred.cr_version == XUCRED_VERSION) {
             std::ostringstream buf;
             buf << ":" << cred.cr_uid << ":";