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!
Stratum: extract_sockaddr: Truncate overlong addresses rather than stack overflow

Thanks to Mick Ayzenberg <mick@dejavusecurity.com> for finding this!
stratum: parse_reconnect(): treat pool-sent URL as untrusted.

Thanks to Mick Ayzenberg <mick@dejavusecurity.com> for reminding
that this existed and highlighting the offender.

Also to Luke-jr for actually fixing this in bfgminer. :D
stratum: parse_notify(): Don't die on malformed bbversion/prev_hash/nbit/ntime.

Might have introduced a memory leak, don't have time to check. :(

Should the other hex2bin()'s be checked?

Thanks to Mick Ayzenberg <mick.dejavusecurity.com> for finding this.
Merge branch 'dejavusec-backport'

Cherry-picked from v5_0.

Fixes for a few security vulnerabilities reported by Mick Ayzenberg of DejaVu Security.