Merge pull request #4 from nikandfor/master

Fix slice out of bounds crash

CONTRIBUTORS

@@ -9,3 +9,4 @@
 Brian Fallik <bfallik@gmail.com>
 Dan Kortschak <dan.kortschak@adelaide.edu.au>
 Jan Mercl <0xjnml@gmail.com>
+Nikifor Seryakov <nikandfor@gmail.com>

btree.go

@@ -450,7 +450,7 @@ func (t *Tree) overflow(p *x, q *d, pi, i int, k interface{} /*K*/, v interface{
 	t.ver++
 	l, r := p.siblings(pi)
 
-	if l != nil && l.c < 2*kd {
+	if l != nil && l.c < 2*kd && i != 0 {
 		l.mvL(q, 1)
 		t.insert(q, i-1, k, v)
 		p.x[pi-1].k = q.d[0].k