[wgsl-in] panic on malformed input

[jimblandy]

The following program causes Naga to panic:

fn S()3-array<f32,0f>()=v}

The crash:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/front/wgsl/mod.rs:1388:83

[SparkyPotato]

Due to #2216, the above code is transformed to

fn S() {
    -array<f32, 0f>() = v
}

(There seems to be another bug in the parser where it doesn't error on the missing ;, somehow array<f32, 0f> = v errors correctly in the parser)

It can be further lowered into

fn S() {
    let x = array<f32, 0f>();
}

The unwrap occurs when lowering zero-value arrays with invalid sizes.