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

@@ -451,9 +451,14 @@ 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 {
-		l.mvL(q, 1)
-		t.insert(q, i-1, k, v)
-		p.x[pi-1].k = q.d[0].k
+		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
 	}