RFR: 8231585: java/lang/management/ThreadMXBean/MaxDepthForThreadInfoTest.java fails with java.lang.NullPointerException

[Daniil Titov]

Please review the change that fixes intermittent failure of java/lang/management/ThreadMXBean/MaxDepthForThreadInfoTest.java

As David noticed (thank you, David, for this analysis) there is no guarantee that all threads found by getAllThreadIds() are still alive by the time we call getThreadInfo() so we have to allow for null array entries.

Testing:  Mach5 tests with Graal on passed 300 times. 

[1] http://cr.openjdk.java.net/~dtitov/8231585/webrev.01/
[2] https://bugs.openjdk.java.net/browse/JDK-8231585  

Best regards,
Daniil