Prevent empty transactions from being added to vtxPrev

CWalletTx::AddSupportingTransactions() was adding empty transaction
to vtxPrev in some cases. Skip over these.

Part one of the solution to #3190. This prevents invalid vtxPrev from
entering the wallet, but not current ones being transmitted.
Merge pull request #3356

d3ef9b0 Prevent empty transactions from being added to vtxPrev (Wladimir J. van der Laan)