Guard agains null ReflogReader if named ref does not exist

Follow up on egit bug 466973.

Change-Id: Idd83d87803e86b25f106dfd725214b5a3ec5171c
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -52,6 +52,7 @@ import java.util.List;
 import org.eclipse.jgit.api.errors.GitAPIException;
 import org.eclipse.jgit.api.errors.InvalidRefNameException;
 import org.eclipse.jgit.api.errors.JGitInternalException;
+import org.eclipse.jgit.api.errors.RefNotFoundException;
 import org.eclipse.jgit.errors.LockFailedException;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.internal.storage.file.ReflogWriter;
@@ -184,6 +185,10 @@ public class StashDropCommand extends GitCommand<ObjectId> {
 		List<ReflogEntry> entries;
 		try {
 			ReflogReader reader = repo.getReflogReader(R_STASH);
+			if (reader == null) {
+				throw new RefNotFoundException(MessageFormat
+						.format(JGitText.get().refNotResolved, stashRef));
+			}
 			entries = reader.getReverseEntries();
 		} catch (IOException e) {
 			throw new JGitInternalException(JGitText.get().stashDropFailed, e);

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

@@ -751,8 +751,11 @@ public abstract class Repository implements AutoCloseable {
 
 	private String resolveReflogCheckout(int checkoutNo)
 			throws IOException {
-		List<ReflogEntry> reflogEntries = getReflogReader(Constants.HEAD)
-				.getReverseEntries();
+		ReflogReader reader = getReflogReader(Constants.HEAD);
+		if (reader == null) {
+			return null;
+		}
+		List<ReflogEntry> reflogEntries = reader.getReverseEntries();
 		for (ReflogEntry entry : reflogEntries) {
 			CheckoutEntry checkout = entry.parseCheckout();
 			if (checkout != null)
@@ -773,6 +776,11 @@ public abstract class Repository implements AutoCloseable {
 		}
 		assert number >= 0;
 		ReflogReader reader = getReflogReader(ref.getName());
+		if (reader == null) {
+			throw new RevisionSyntaxException(
+					MessageFormat.format(JGitText.get().reflogEntryNotFound,
+							Integer.valueOf(number), ref.getName()));
+		}
 		ReflogEntry entry = reader.getReverseEntry(number);
 		if (entry == null)
 			throw new RevisionSyntaxException(MessageFormat.format(