Encountering OP_VERIF and OP_VERNOTIF in execution is always an error.

I honestly thought we already handled this, but some tests dhill is
working on shows that we didn't.

opcode.go

@@ -862,6 +862,22 @@ type parsedOpcode struct {
 	opfunc func(op parsedOpcode, s Script) error
 }
 
+// The following opcodes are always illegal when passed over by the program
+// counter even if in a non-executed branch. (it isn't a coincidence that they
+// are conditionals).
+func (pop *parsedOpcode) alwaysIllegal() bool {
+	switch pop.opcode.value {
+	case OP_VERIF:
+		return true
+	case OP_VERNOTIF:
+		return true
+	default:
+		return false
+	}
+}
+
+// The following opcode are conditional and thus change the conditional
+// execution stack state when passed.
 func (pop *parsedOpcode) conditional() bool {
 	switch pop.opcode.value {
 	case OP_IF:

script.go

@@ -52,6 +52,11 @@ var (
 	// is encountered.
 	StackErrReservedOpcode = errors.New("Reserved Opcode")
 
+	// StackErrAlwaysIllegal is returned when an opcode marked as always
+	// illegal is encountered. Currently this is just OP_VERIF and
+	// OP_VERNOTIF.
+	StackErrAlwaysIllegal = errors.New("Always Illlegal instruction encountered")
+
 	// StackErrEarlyReturn is returned when OP_RETURN is executed in the
 	// script.
 	StackErrEarlyReturn = errors.New("Script returned early")
@@ -602,7 +607,9 @@ func (m *Script) Step() (done bool, err error) {
 	if m.condStack[0] != OpCondTrue {
 		// some opcodes still 'activate' if on the non-executing side
 		// of conditional execution
-		if opcode.conditional() {
+		if opcode.alwaysIllegal() {
+			return true, StackErrAlwaysIllegal
+		} else if opcode.conditional() {
 			executeInstr = true
 		} else {
 			executeInstr = false