RFR: 8274621: NullPointerException because listenAddress[0] is null

[Alex Menkov]

The change fixes ProcessTools.startProcess "warmup predicate" synchronization issue.
Initially the predicate was called only for STDOUT;
>From jdk8 it's called for STDERR too (but ProcessTools javadoc was not updated).
The fix keeps existing functionality as is (as we have this behavior for a long time and we have tests which expect STDERR output), but adds synchronization to avoid calling predicate after previous call returned "true".
Also updated javadoc to reflect actual behavior.

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Commit messages:
 - Added synchronization between stdout and stderr warmup predicates

Changes: https://git.openjdk.java.net/jdk/pull/5830/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5830&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274621
  Stats: 14 lines in 1 file changed: 4 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5830.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5830/head:pull/5830

PR: https://git.openjdk.java.net/jdk/pull/5830