Explicitly unpoison FDs for MSan.

MSan doesn't handle inline assembly that's used by FD_ZERO causing a
false positive.

R=earthdok@chromium.org, henrike@webrtc.org
BUG=chromium:344505

Review URL: https://webrtc-codereview.appspot.com/25799004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7388 4adac7df-926f-26a2-2b94-8c16560cd09d