DERBY-5960 VirtualRandomAccessFile.close() is not idempotent

Merge revision 1402111 from trunk.
Contributed by Knut Anders Hatlen




git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.8@1509794 13f79535-47bb-0310-9956-ffa450edef68