txscript/engine: Check ps2h push before parsing script
txscript/engine: Check ps2h push before parsing script

This moves the check for non push-only pay-to-script-hash signature
scripts before the script parsing logic when creating a new engine
instance to avoid the extra overhead in the error case.

btcsuite/btcd,ltcsuite/ltcd,davecgh/btcd,tb00/btcd,btcsuite/btcd,ltcsuite/ltcd,tb00/btcd,davecgh/btcd