Index out of bounds in lower/mod.rs

[wgslfuzz]

On git commit 5b65f11 naga panics with the following backtrace when executing naga input.wgsl output.hlsl

thread 'main' panicked at src/front/wgsl/lower/mod.rs:1705:49:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0: rust_begin_unwind
             at /rustc/030e4d382f1df30240408540f25cd1ccc8dbbf50/library/std/src/panicking.rs:619:5
   1: core::panicking::panic_fmt
             at /rustc/030e4d382f1df30240408540f25cd1ccc8dbbf50/library/core/src/panicking.rs:72:14
   2: core::panicking::panic_bounds_check
             at /rustc/030e4d382f1df30240408540f25cd1ccc8dbbf50/library/core/src/panicking.rs:180:5
   3: naga::front::wgsl::lower::Lowerer::expression_for_reference
   4: naga::front::wgsl::lower::Lowerer::expression

fn f() -> vec4<f32> {
    return bitcast<mat3x4<f32>>(0.0);
}