gcs: check to see if sigScript exists before adding it

gcs/builder/builder.go

@@ -328,10 +328,12 @@ func BuildExtFilter(block *wire.MsgBlock) (*gcs.Filter, error) {
 		// Skip the inputs for the coinbase transaction
 		if i != 0 {
 			for _, txIn := range tx.TxIn {
+				if txIn.SignatureScript != nil {
 					b.AddScript(txIn.SignatureScript)
 				}
 			}
 		}
+	}
 
 	return b.Build()
 }