miniscript_parse: crash with `ttvtvtvtvtvtvtv:after(2)`

```sh
echo -n "ttvtvtvtvtvtvtv:after(2)" > embit.crash
export CXXFLAGS="-DBITCOIN_CORE -DEMBIT -DRUST_MINISCRIPT"
make 
FUZZ=miniscript_parse ./bitcoinfuzz embit.crash
```

Bitcoin Core and rust-miniscript return false and embit returns true.