fix ubsan: bitcoin-tx: not initialize context before IsFullyValid
fix ubsan: bitcoin-tx: not initialize context before IsFullyValid

>>> backports bitcoin@321bbc2079e2dc3648ae4eaf845f5b71d30b423a
Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX