Signrawtransaction shouldn't require redeemScript for non-p2sh inputs

The redeemScript functionality broke plain offline signing, this
change makes it only look for that parameter when signing a p2sh
input.

Shorter version of https://github.com/bitcoin/bitcoin/pull/2264

Tested by creating two chained, not-broadcast raw transactions, verifying that not providing redeemScript for the second fails without this change, succeeds with this change.