ObjectWalk: Add null check before skip tree.

currVisit could be null if a blob is marked as start point in
ObjectWalk. Add null check before skipping current tree.

Change-Id: Ic5d876fe2800f3373d136979be6c27d1bbd38dc1
Signed-off-by: Yunjie Li <yunjieli@google.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/ObjectWalkTest.java

@@ -216,4 +216,12 @@ public class ObjectWalkTest extends RevWalkTestCase {
 		assertSame(rw.lookupBlob(bId), objw.nextObject());
 		assertNull(objw.nextObject());
 	}
+
+	@Test
+	public void testSkipTreeWhenStartFromBlob() throws Exception {
+		final RevBlob f1 = blob("1");
+		objw.markStart(f1);
+		assertSame(f1, objw.nextObject());
+		objw.skipTree();
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/ObjectWalk.java

@@ -364,8 +364,10 @@ public class ObjectWalk extends RevWalk {
 	 * @since 5.4
 	 */
 	public void skipTree() {
+		if (currVisit != null) {
 			currVisit.ptr = currVisit.buf.length;
 		}
+	}
 
 	/**
 	 * Pop the next most recent object.