Fix possible NPE in Repository.resolve

Change-Id: I9b19cd90b338a4c3cae5661c658526b7e98c95d1
Signed-off-by: Marc Strapetz <marc.strapetz@syntevo.com>‌

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

 		try {
 			Object resolved = resolve(rw, revstr);
 			if (resolved instanceof String) {
-				return getRef((String) resolved).getLeaf().getObjectId();
+				final Ref ref = getRef((String)resolved);
+				return ref != null ? ref.getLeaf().getObjectId() : null;
 			} else {
 				return (ObjectId) resolved;
 			}