CVE-2022-49307 tty: synclink_gt: Fix null-pointer-dereference in slgt_clean()

In the Linux kernel, the following vulnerability has been resolved: tty: synclinkgt: Fix null-pointer-dereference in slgtclean When the driver fails at allochdlcdev, and then we remove the driver module, we will get the following splat: 25.065966 general protection fault, probably for non-canonic...