Fix slice out of bounds crash

There was crash when we try insert element to full data struct at position i=0
when left neighbour data struct was not full

btree.go

@@ -451,12 +451,17 @@ func (t *Tree) overflow(p *x, q *d, pi, i int, k interface{} /*K*/, v interface{
 	l, r := p.siblings(pi)
 
 	if l != nil && l.c < 2*kd {
+		if i > 0 {
 			l.mvL(q, 1)
 			t.insert(q, i-1, k, v)
 			p.x[pi-1].k = q.d[0].k
 			return
 		}
 
+		t.insert(l, l.c, k, v)
+		return
+	}
+
 	if r != nil && r.c < 2*kd {
 		if i < 2*kd {
 			q.mvR(r, 1)