tty: fix possible null-ptr-defer in spk_ttyio_release

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.

Fixes: 4f2a81f3a882 ("speakup: Reference synth from tty and tty from synth")
Cc: stable <stable@kernel.org>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Link: https://lore.kernel.org/r/20221202060633.217364-1-cuigaosheng1@huawei.com
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>