api/asset: fix input sig data on combine/Trade
api/asset: fix input sig data on combine/Trade

Previously the hash to sign was only being created for the individual
build call, but it needs to be for the combined txs. Moving the call
to this step fixes invalid signature errors during the finalize step.

Closes chain/chainprv#393.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@jeffomatic

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain