ubsan.test_longjmp_zero is failing

For some reason this test fails, but only under ubsan:

```
Output:
start
longjmp
/usr/local/google/home/sbc/dev/wasm/emscripten/test/core/test_longjmp_zero.c:25:3: runtime error: execution reached an unreachable program point
```

Its like ubsan is inserting the its "does not return" check incorrectly somehow, because this function clearly isn't returning in practice.