msgr: fail on sd < 0
msgr: fail on sd < 0

I saw a case where poll(2) was blocking despite being passed an fd of -1.
Since that's clearly invalid, we can fail tcp_{read,write} before that
point.

Weird.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph