nfs server fails with stacktrace on craete with krb5

[kofemann]

When mounting with krb5 and user mapping returns NOBODY as a result of missing mapping, then
a create request will fail with following stacktrace:

22 Jun 2016 13:04:02 (NFS-dcache-lab000) [] Unhandled exception:
java.util.NoSuchElementException: Subject has no UID
    at org.dcache.auth.Subjects.getUid(Subjects.java:164) ~[dcache-common-2.17.0-SNAPSHOT.jar:2.17.0-SNAPSHOT]
    at org.dcache.chimera.nfsv41.door.ChimeraVfs.create(ChimeraVfs.java:132) ~[dcache-nfs-2.17.0-SNAPSHOT.jar:2.17.0-SNAPSHOT]
    at org.dcache.nfs.vfs.VfsCache.create(VfsCache.java:156) ~[nfs4j-core-0.12.2.jar:0.12.2]
    at org.dcache.nfs.vfs.PseudoFs.create(PseudoFs.java:155) ~[nfs4j-core-0.12.2.jar:0.12.2]
    at org.dcache.nfs.v4.OperationOPEN.process(OperationOPEN.java:158) ~[nfs4j-core-0.12.2.jar:0.12.2]
    at org.dcache.chimera.nfsv41.door.proxy.ProxyIoMdsOpFactory$1.lambda$process$0(ProxyIoMdsOpFactory.java:53) ~[dcache-nfs-2.17.0-SNAPSHOT.jar:2.17.0-SNAPSHOT]
    at org.dcache.chimera.nfsv41.door.proxy.ProxyIoMdsOpFactory$1$$Lambda$72/1766112330.run(Unknown Source) ~[na:na]
    at java.security.AccessController.doPrivileged(Native Method) ~[na:1.8.0_31]
    at javax.security.auth.Subject.doAs(Subject.java:360) ~[na:1.8.0_31]
    at org.dcache.chimera.nfsv41.door.proxy.ProxyIoMdsOpFactory$1.process(ProxyIoMdsOpFactory.java:50) ~[dcache-nfs-2.17.0-SNAPSHOT.jar:2.17.0-SNAPSHOT]
    at org.dcache.nfs.v4.NFSServerV41.NFSPROC4_COMPOUND_4(NFSServerV41.java:152) ~[nfs4j-core-0.12.2.jar:0.12.2]
    at org.dcache.nfs.v4.xdr.nfs4_prot_NFS4_PROGRAM_ServerStub.dispatchOncRpcCall(nfs4_prot_NFS4_PROGRAM_ServerStub.java:48) [nfs4j-core-0.12.2.jar:0.12.2]
    at org.dcache.xdr.RpcDispatcher$1.run(RpcDispatcher.java:82) [oncrpc4j-core-2.5.3.jar:na]
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:591) [grizzly-framework-2.3.24.jar:2.3.24]
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:571) [grizzly-framework-2.3.24.jar:2.3.24]
    at java.lang.Thread.run(Thread.java:745) [na:1.8.0_31]