Return a min/max version of zero if set to zero.

Return a min/max version of zero if set to zero.

OpenSSL's SSL{_CTX,}_get_{min,max}_proto_version() return a version of zero
if the minimum or maximum has been set to zero (which means the minimum or
maximum version supported by the method). Previously we returned the
minimum or maximum version supported by the method, instead of zero. Match
OpenSSL's behaviour by using shadow variables.

Discussed with tb@