Fix possible NPE in LsUserRefs

If a RepositoryNotFoundException is thrown 'repo' may not be set.

Change-Id: Ie405141117dd2941251ae176777f0c460ab518dd
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-sshd/src/main/java/com/google/gerrit/sshd/commands/LsUserRefs.java

@@ -109,7 +109,9 @@ public class LsUserRefs extends SshCommand {
       throw new UnloggedFailure("fatal: Error opening: '"
           + projectControl.getProject().getNameKey());
     } finally {
-      repo.close();
+      if (repo != null) {
+        repo.close();
+      }
     }
   }
 }