RFR JDK-8149769: Null pointer exception in ZipFileSystemProvider

[Steve Drach]

>> Please review this simple fix to ZipFileSystemProvider.  The issue is JDK-8149769 <https://bugs.openjdk.java.net/browse/JDK-8149769>.  I didn’t do a webrev but instead provide the following patch.
>> 
>> 
> This looks okay. Can one of the existing tests be updated to cover this case?

Yes, almost any of them can be updated.  There is one, ZipFSTester.java that seems to be associated with a lot of bugs, so maybe that one.  Or I can put it in MultiReleaseJarTest, which makes sense because the code it tests was the code that caused the bug.  Any preferences?