Fix #683, if a user tries to preview a transaction without any local state (headers, etc) the local height will be -1, and when used as the locktime will choke the hex serialisation.  The change made sets the locktime to 0, if the local blockheight is -1, which should mean the tx can be broadcast and mined immediately.
Fix #683, if a user tries to preview a transaction without any local state (headers, etc) the local height will be -1, and when used as the locktime will choke the hex serialisation.  The change made sets the locktime to 0, if the local blockheight is -1, which should mean the tx can be broadcast and mined immediately.

fyookball/electrum,fyookball/electrum,fyookball/electrum