RFR: 8214202: DirtyCardQueueSet::get_completed_buffer should not clear _process_completed

[Kim Barrett]

Please review this small fix for get_completed_buffer.  As discussed
in the CR, it should not be setting _process_completed to false when
the stop_at limit has been reached.

While I was there, simplified the code a little, eliminating an
unnecessary NULL test.

Also fixed a mistaken use of _process_completed in an assert message,
spotted while examining all uses of that variable.

CR:
https://bugs.openjdk.java.net/browse/JDK-8214202

Webrev:
http://cr.openjdk.java.net/~kbarrett/8214202/open.00/

Testing:
mach5 tier1-3.