Redundant assert "compiler should always document failure: %s" with possible UB

JDK-8303951 added the following code: https://github.com/openjdk/jdk/pull/13038/files#diff-2e74481e557cbe87170a56a6e592eea33bb59019926e1c32bebcfaf5b571bb53R2280

```
     if (!ci_env.failing() && !task->is_success()) {
+ assert(ci_env.failure_reason() != nullptr, "expect failure reason");
+ assert(false, "compiler should always document failure: %s", ci_env.failure_reason());
```

`ciEnv` functions:
```
bool failing() const { return _failure_reason.get() != nullptr; }
const char* failure_reason() const { return _failure_reason.get(); }
```
So the code above is:
```
    if (_failure_reason.get() == nullptr && !task->is_success()) {
      assert(_failure_reason.get() != nullptr, "expect failure reason");
      assert(false, "compiler should always document failure: %s", ci_env.failure_reason());
```

The condition `_failure_reason.get() != nullptr` is always false in the assert because we can get to it only if `_failure_reason.get() == nullptr`. So it will be triggered. The second assert is redundant.

The code above should be changed to:
```
   if (!ci_env.failing() && !task->is_success()) {
      assert(ci_env.failure_reason() != nullptr, "compiler should always document failure");
      // The compiler elected, without comment, not to register a result.
```

`assert(false, "compiler should always document failure: %s", ci_env.failure_reason()); ` has UB because `ci_env.failure_reason()` is `nullptr`.