Assertion failure in ComputeType when processing script OP_0 OP_0 OP_EQUAL

The following assertion failure in `ComputeType` is hit when passing the script `OP_0 OP_0 OP_EQUAL` (`00 00 87`) to `miniscript::FromScript`.

```
bitcoin/script/miniscript.cpp:54: 
    miniscript::Type miniscript::internal::ComputeType(…):
    Assertion `k > 1 && k < n_subs' failed.
```