Ignore clippy::out_of_bounds_indexing error (#252)

```
error: index is out of bounds
   --> src/util.rs:134:17
    |
134 |         None => [][1],
    |                 ^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#out_of_bounds_indexing
    = note: `#[deny(clippy::out_of_bounds_indexing)]` on by default

error: index is out of bounds
   --> src/util.rs:146:17
    |
146 |         None => [][1],
    |                 ^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#out_of_bounds_indexing
```

Author: taiki-e

src/util.rs

@@ -127,7 +127,7 @@ pub(crate) const fn cast_ptr<T>(n: &T) -> *const T {
 ///
 /// This is a workaround for the lack of panic-in-const in older
 /// toolchains.
-#[allow(unconditional_panic)]
+#[allow(unconditional_panic, clippy::out_of_bounds_indexing)]
 pub(crate) const fn unwrap_u32(t: Option<u32>) -> u32 {
     match t {
         Some(v) => v,
@@ -139,7 +139,7 @@ pub(crate) const fn unwrap_u32(t: Option<u32>) -> u32 {
 ///
 /// This is a workaround for the lack of panic-in-const in older
 /// toolchains.
-#[allow(unconditional_panic)]
+#[allow(unconditional_panic, clippy::out_of_bounds_indexing)]
 pub(crate) const fn unwrap_nonzero(t: Option<NonZeroU32>) -> NonZeroU32 {
     match t {
         Some(v) => v,