crypto: do not move half-filled write head

Might cause write head running over read head, when there were no
allocation and `Commit()` was called. Source of at least one test
failure on windows (`simple/test-https-drain.js`).
uv: Upgrade to v0.11.5
NodeConf 2013: example 1
NodeConf 2013: example 2
NodeConf 2013: exercize
NodeConf 2013: exercize extra credit