fix: remove item may changed clone btree; (i+1) children has new cow, but do not copy

Signed-off-by: zhangchuanqing1 <zhangchuanqing1@jd.com>

btree.go

diff --git a/btree.go b/btree.go
index 969b910..6f5184f 100644
--- a/btree.go
+++ b/btree.go
@@ -479,7 +479,7 @@
 		child := n.mutableChild(i)
 		// merge with right child
 		mergeItem := n.items.removeAt(i)
-		mergeChild := n.children.removeAt(i + 1)
+		mergeChild := n.children.removeAt(i + 1).mutableFor(n.cow)
 		child.items = append(child.items, mergeItem)
 		child.items = append(child.items, mergeChild.items...)
 		child.children = append(child.children, mergeChild.children...)