This is something a well-behaving STUN server shouldn't do, but we shouldn't crash if it does happen. Also adding helper function for the common operation of extracting just the error code out of a STUN packet. BUG=chromium:708469 Review-Url: https://codereview.webrtc.org/2837133003 Cr-Commit-Position: refs/heads/master@{#17892}