Questionable 'arity mismatch'

This example fails with a 'arity mismatch' in the ml-proto. It is accepted by sexp-wasm.

```
(module
 (func $f1 (result i32)
   (block $l0
     (br_if (i32.const 1) $l0
       (block $l1
         (br $l1 (i32.const 1))))
     (i32.const 1))))
```
