Merge "Prevent NPE if ref can't be resolved when executing ReflogCommand" into stable-3.6

org.eclipse.jgit/src/org/eclipse/jgit/api/ReflogCommand.java

 
 import org.eclipse.jgit.api.errors.GitAPIException;
 import org.eclipse.jgit.api.errors.InvalidRefNameException;
+import org.eclipse.jgit.api.errors.RefNotFoundException;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ReflogEntry;
 
 		try {
 			ReflogReader reader = repo.getReflogReader(ref);
+			if (reader == null)
+				throw new RefNotFoundException(MessageFormat.format(
+						JGitText.get().refNotResolved, ref));
 			return reader.getReverseEntries();
 		} catch (IOException e) {
 			throw new InvalidRefNameException(MessageFormat.format(