Fix Fiber#alive? regression.

In JRuby 1.7.10:
```ruby
require 'fiber'

fiber = Fiber.new {}

fiber.alive? #=> true
fiber.resume
fiber.alive? #=> false
```

In JRuby 1.7.11 through 1.7.15:
```ruby
require 'fiber'

fiber = Fiber.new {}

fiber.alive? #=> true
fiber.resume
fiber.alive? #=> true
```

It looks like this is [already fixed on master](https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/ext/fiber/ThreadFiber.java#L224).

Closes #1789.