Fix Cursor.Last() on empty buckets.

@tv42 reported that creating a cursor on an empty bucket and then calling
Cursor.Last() causes an index out of range error and panics. This commit
adds a check for the page's item count being greater than zero.

Fixes #63.

1 files changed, 1 insertions, 1 deletions

diff --git a/cursor.go b/cursor.go
index 244c915..d66f1c1 100644
--- a/cursor.go
+++ b/cursor.go
@@ -235,7 +235,7 @@ func (c *Cursor) nsearch(key []byte) {
 // keyValue returns the key and value of the current leaf element.
 func (c *Cursor) keyValue() ([]byte, []byte) {
 	ref := &c.stack[len(c.stack)-1]
-	if ref.index >= ref.count() {
+	if ref.count() == 0 || ref.index >= ref.count() {
 		return nil, nil
 	}