Typedarray test sometimes unwraps an error result

[jdm]

From an appveyor run:

running 2 tests
test typedarray ... FAILED
test typedarray_update_panic ... ok
failures:
---- typedarray stdout ----
    thread 'typedarray' panicked at 'called `Result::unwrap()` on an `Err` value: ()', libcore\result.rs:945:5
failures:
    typedarray
test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
error: test failed, to rerun pass '--test typedarray'
Command exited with code 101