Fix RetryRule

[alb-i986]

1. fix NPE

The following unit test shows the bug:

public class RetryRuleTest {

    @Test
    public void asd() throws Throwable {
        RetryRule sut = new RetryRule(1);

        Description desc = Description.createTestDescription("class", "name");
        Statement failingTest = new Fail(new ExpectedException("simulated failure"));

        try {
            sut.apply(failingTest, desc).evaluate();
        } catch (ExpectedException e) {
            // expected
        }
    }

    private static class ExpectedException extends RuntimeException {
        public ExpectedException(String message) {
            super(message);
        }
    }
}

1. refactor: no need to use AtomicInteger

[ndmanvar]

@mehmetg Could you please review?

[mehmetg]

Wouldn't adding a "=" to the first comparison fix or pulling out the assignment to caughtThrowable from within the if statement? Just like the refactored code does.

[mehmetg]

The point is valid, just don't see a reason for the rewrite this fix can be handled, with a single line change.
AtomicInteger use is not required, but this is not a real concern here.