DSLX: Segfault on malformed code

The following code shouldn't...parse, I guess, but does, and causes a segfault (in type deduction):
```
fn linearize_block(block: Block) -> uN[128] {
block[0] as u32 ++ block[1] as u32 ++ block[2] as u32 ++ block[3] as u32
}

fn block_to_xfer_chunk(block: Block) -> XferChunk {
uN[384]:0 ++ linearize_block
}
```

This is invalid code since `linearize_block` is a function, not a value.