break outside loop causes compiler assertion

```haxe
function main() {
@:coroutine function f() {
break;
}
}
```

```
Fatal error: exception Failure("hd")
Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
Called from Stdlib__List.hd in file "list.ml" (inlined), line 30, characters 10-23
Called from CoroFromTexpr.expr_to_coro.loop in file "src/coro/coroFromTexpr.ml", line 217, characters 44-65
Called from CoroFromTexpr.expr_to_coro.loop_assign in file "src/coro/coroFromTexpr.ml", line 380, characters 11-24
```

The typer doesn't error hard enough for the coro transformer to not run, and it then gets confused.

Reproduce the Haxe snippet and inspect src/coro/coroFromTexpr.ml around lines 217 and 380, where the assertion is reported. Check how the typer handles break outside a loop before the coroutine transformer runs. Done means the invalid program produces a compiler error instead of the coroutine assertion.

Written by the indexing model from the issue text.