use CString with SetTlsExtHostName

i guess we can't rely on there being a null byte after the memory for a
byte array. in retrospect it seems really stupid to think there would
be.

go me

Change-Id: I35ab6704cefbfbde064906aa682985574cb7e034
Merge pull request #3 from thepaul/master

use CString with SetTlsExtHostName