tty: fix possible null-ptr-defer in spk_ttyio_release

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

tty: fix possible null-ptr-defer in spk_ttyio_release

Run the following tests on the qemu platform:

syzkaller:~# modprobe speakup_audptr
input: Speakup as /devices/virtual/input/input4
initialized device: /dev/synth, node (MAJOR 10, MINOR 125)
speakup 3.1.6: initialized
synth name on entry is: (null)
synth probe

spk_ttyio_initialise_ldisc failed because tty_kopen_exclusive returned
failed (errno -16), then remove the module, we will get a null-ptr-defer
problem, as follow:

syzkaller:~# modprobe -r speakup_audptr
releasing synth audptr
BUG: kernel NULL pointer dereference, address: 0000000000000080
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP PTI
CPU: 2 PID: 204 Comm: modprobe Not tainted 6.1.0-rc6-dirty #1
RIP: 0010:mutex_lock+0x14/0x30
Call Trace:
<TASK>
spk_ttyio_release+0x19/0x70 [speakup]
synth_release.part.6+0xac/0xc0 [speakup]
synth_remove+0x56/0x60 [speakup]
__x64_sys_delete_module+0x156/0x250
? fpregs_assert_state_consistent+0x1d/0x50
do_syscall_64+0x37/0x90
entry_SYSCALL_64_after_hwframe+0x63/0xcd
</TASK>
Modules linked in: speakup_audptr(-) speakup
Dumping ftrace buffer:

in_synth->dev was not initialized during modprobe, so we add check
for in_synth->dev to fix this bug.