hg: jdk/sandbox: 8241883: (zipfs) SeekableByteChannel:close followed by SeekableByteChannel:close will throw an NPE coverage

[chris.hegarty at oracle.com]

Changeset: 909946b881c7
Author:    lancea
Date:      2020-04-10 14:00 -0400
URL:       https://hg.openjdk.java.net/jdk/sandbox/rev/909946b881c7

8241883: (zipfs) SeekableByteChannel:close followed by SeekableByteChannel:close will throw an NPE coverage
Reviewed-by: clanger, alanb

! src/jdk.zipfs/share/classes/jdk/nio/zipfs/ByteArrayChannel.java
! src/jdk.zipfs/share/classes/jdk/nio/zipfs/ZipFileSystem.java
! test/jdk/jdk/nio/zipfs/testng/test/ChannelTests.java