if split is too small, such as 0, use it to calculate continued will out of bound map

#syz test: upstream 55027e689933

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index a630b27a4cc6..0a111274dc4a 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2043,7 +2043,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
 		split = count/2;
 
 	hash2 = map[split].hash;
-	continued = hash2 == map[split - 1].hash;
+	continued = split > 0 ? hash2 == map[split - 1].hash : 0;
 	dxtrace(printk(KERN_INFO "Split block %lu at %x, %i/%i\n",
 			(unsigned long)dx_get_block(frame->at),
 					hash2, split, count-split));

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-and-tested-by: syzbot+ae688d469e36fb5138d0@syzkaller.appspotmail.com

Tested on:

commit:         55027e68 Merge tag 'input-for-v6.10-rc5' of git://git...
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=16bb8761980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=67463c0717b8d4ca
dashboard link: https://syzkaller.appspot.com/bug?extid=ae688d469e36fb5138d0
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=10754c81980000

Note: testing is done by a robot and is best-effort only.