Possible to hit assertion failure in malleability check

Adding this to `miniscript/mod.rs` results in a panic.
```
#[test]
fn test_regression_29() {
let _ = Descriptor::::from_str("eltr(,thresh(1,spk_eq(,00)))");
}
```
```
assertion failed: self.mall.non_malleable || self.corr.input != Input::Zero', /store/home/apoelstra/code/ElementsProject/elements-miniscript/fuzzing/src/miniscript/types/mod.rs:556:9
```