[Rust] Fix clippy error with Rust 1.74.0

    Checking zstd v0.12.4
    Checking hello-wasm v0.1.0 (/home/martin/git/apache/avro/lang/rust/wasm-demo)
error: this loop never actually loops
  --> avro_derive/tests/derive.rs:72:9
   |
72 | /         for res in reader {
73 | |             match res {
74 | |                 Ok(value) => {
75 | |                     return from_value::<T>(&value).unwrap();
...  |
78 | |             }
79 | |         }
   | |_________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#never_loop
   = note: `-D clippy::never-loop` implied by `-D clippy::all`
   = help: to override `-D clippy::all` add `#[allow(clippy::never_loop)]`
help: if you need the first element of the iterator, try writing
   |
72 |         if let Some(res) = reader.next() {
   |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~