[PATCH] block: fix build waring

[Zhiwei Jiang <elish.jiang@ucloud.cn>]

when i compile this file with some error message
../block.c: In function ‘bdrv_replace_node_common’:
../block.c:4903:9: error: ‘to_cow_parent’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
         bdrv_remove_filter_or_cow_child(to_cow_parent, tran);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Zhiwei Jiang <elish.jiang@ucloud.cn>
---
 block.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 3f456892d0..08f29e6b65 100644
--- a/block.c
+++ b/block.c
@@ -4866,7 +4866,7 @@ static int bdrv_replace_node_common(BlockDriverState *from,
     Transaction *tran = tran_new();
     g_autoptr(GHashTable) found = NULL;
     g_autoptr(GSList) refresh_list = NULL;
-    BlockDriverState *to_cow_parent;
+    BlockDriverState *to_cow_parent = NULL;
     int ret;
 
     if (detach_subchain) {
-- 
2.25.1