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

In the Linux kernel, the following vulnerability has been resolved:

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

When the driver fails at alloc_hdlcdev(), and then we remove the driver

module, we will get the following splat:

[ 25.065966] general protection fault, probably for non-canonical address 0xdffffc0000000182: 0000 [#1] PREEMPT SMP KASAN PTI

[ 25.066914] KASAN: null-ptr-deref in range [0x0000000000000c10-0x0000000000000c17]

[ 25.069262] RIP: 0010:detach_hdlc_protocol+0x2a/0x3e0

[ 25.077709] Call Trace:

[ 25.077924] <TASK>

[ 25.078108] unregister_hdlc_device+0x16/0x30

[ 25.078481] slgt_cleanup+0x157/0x9f0 [synclink_gt]

Fix this by checking whether the 'info->netdev' is a null pointer first.