CVE-2024-49868 btrfs: fix a NULL pointer dereference when failed to start a new trasacntion

In the Linux kernel, the following vulnerability has been resolved: btrfs: fix a NULL pointer dereference when failed to start a new trasacntion BUG Syzbot reported a NULL pointer dereference with the following crash: FAULTINJECTION: forcing a failure. starttransaction+0x830/0x1670...