OP_NOTIF without OP_IF marked invalid

E.g.:

```
OP_1 OP_NOTIF OP_3 OP_ENDIF
```

is marked as invalid, but I think it is allowed, e.g. with btcdeb:

```
./btcdeb '[OP_1 OP_NOTIF OP_3 OP_ENDIF]'
```

seems valid.

WIth OP_IF it is valid in both this project and btcdeb:

```
OP_1 OP_IF OP_2 OP_NOTIF OP_3 OP_ENDIF
```
