txscript: Only do CSV txver check if enabled.
txscript: Only do CSV txver check if enabled.

The CSV consensus rules dictate that the opcode fails when the
transaction version is not at least version 2, however that only applies
if the disable flag is not set in the sequence.

This is not an issue at the current time because we do not yet enforce
CSV at a consensus level, however, I noticed this discrepancy when doing
a thorough audit of the CSV paths due to the ongoing work to add full
consensus-enforced CSV support.

As a result, this must be merged prior to enabling consensus enforcement
for CSV or it would open up the potential for a hard fork.

aakselrod/btcd,adiabat/btcd,adiabat/btcd,cqswxj2013/btcd,wallclockbuilder/btcd,ppcsuite/ppcd,ltcsuite/ltcd,dajohi/btcd,Stash-Crypto/btcd,dajohi/btcd,tb00/btcd,ppcsuite/ppcd,ltcsuite/ltcd,tb00/btcd,dajohi/btcd,Stash-Crypto/btcd,jongillham/btcd,Roasbeef/btcd,btcsuite/btcd,jongillham/btcd,btcsuite/btcd,wallclockbuilder/btcd,Roasbeef/btcd,davecgh/btcd,aakselrod/btcd,davecgh/btcd,cqswxj2013/btcd