RFR(xxs): 8202303: LogStream should autoflush on destruction

[Thomas Stüfe]

Hi all,

please review this tiny change:

https://bugs.openjdk.java.net/browse/JDK-8202303
http://cr.openjdk.java.net/~stuefe/webrevs/8202303-logstream-autoflush/webrev.00/webrev/

Basically, LogStream should not assert upon desctruction when having
buffered unfinished lines; instead, it should autoflush.

(Note: the changes to the TestLineBufferAllocation test are just to
keep this test in line with the rest of the tests in regards to how
the fake log file is handled).

More details in my earlier mail to hs-runtime:
http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2018-April/027592.html

Thanks, Thomas