Stratum: extract_sockaddr: Truncate overlong addresses rather than stack overflow

Thanks to Mick Ayzenberg <mick@dejavusecurity.com> for finding this!
Bugfix: initiate_stratum: Ensure extranonce2 size is not negative (which could lead to exploits later as too little memory gets allocated)

Thanks to Mick Ayzenberg <mick@dejavusecurity.com> for finding this!
Merge commit 'ff7f301' into bfgminer-3.5.x