Fix unused result that is `#[must_use]` (#4663)

Fixes this compiler warning:

```
warning: unused return value of `Box::<T>::from_raw` that must be used
   --> crates/bench-api/src/lib.rs:351:9
    |
351 |         Box::from_raw(state as *mut BenchState);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
```

crates/bench-api/src/lib.rs

@@ -348,7 +348,7 @@ pub extern "C" fn wasm_bench_create(
 pub extern "C" fn wasm_bench_free(state: *mut c_void) {
     assert!(!state.is_null());
     unsafe {
-        Box::from_raw(state as *mut BenchState);
+        drop(Box::from_raw(state as *mut BenchState));
     }
 }