setup_fritz() in avmfritz.c might fail with -EIO and in this case the
isac.type and isac.write_reg is not initialized and remains 0(NULL).
A subsequent call to isac_release() will dereference isac->write_reg and
crash.

[    1.737444] BUG: kernel NULL pointer dereference, address: 0000000000000000
[    1.737809] #PF: supervisor instruction fetch in kernel mode
[    1.738106] #PF: error_code(0x0010) - not-present page
[    1.738378] PGD 0 P4D 0
[    1.738515] Oops: 0010 [#1] SMP NOPTI
[    1.738711] CPU: 0 PID: 180 Comm: systemd-udevd Not tainted 5.12.0-rc2+ #78
[    1.739077] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-48-gd9c812dda519-p
rebuilt.qemu.org 04/01/2014
[    1.739664] RIP: 0010:0x0
[    1.739807] Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
[    1.740200] RSP: 0018:ffffc9000027ba10 EFLAGS: 00010202
[    1.740478] RAX: 0000000000000000 RBX: ffff888102f41840 RCX: 0000000000000027
[    1.740853] RDX: 00000000000000ff RSI: 0000000000000020 RDI: ffff888102f41800
[    1.741226] RBP: ffffc9000027ba20 R08: ffff88817bc18440 R09: ffffc9000027b808
[    1.741600] R10: 0000000000000001 R11: 0000000000000001 R12: ffff888102f41840
[    1.741976] R13: 00000000fffffffb R14: ffff888102f41800 R15: ffff8881008b0000
[    1.742351] FS:  00007fda3a38a8c0(0000) GS:ffff88817bc00000(0000) knlGS:0000000000000000
[    1.742774] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    1.743076] CR2: ffffffffffffffd6 CR3: 00000001021ec000 CR4: 00000000000006f0
[    1.743452] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    1.743828] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[    1.744206] Call Trace:
[    1.744339]  isac_release+0xcc/0xe0 [mISDNipac]
[    1.744582]  fritzpci_probe.cold+0x282/0x739 [avmfritz]
[    1.744861]  local_pci_probe+0x48/0x80
[    1.745063]  pci_device_probe+0x10f/0x1c0
[    1.745278]  really_probe+0xfb/0x420
[    1.745471]  driver_probe_device+0xe9/0x160
[    1.745693]  device_driver_attach+0x5d/0x70
[    1.745917]  __driver_attach+0x8f/0x150
[    1.746123]  ? device_driver_attach+0x70/0x70
[    1.746354]  bus_for_each_dev+0x7e/0xc0
[    1.746560]  driver_attach+0x1e/0x20
[    1.746751]  bus_add_driver+0x152/0x1f0
[    1.746957]  driver_register+0x74/0xd0
[    1.747157]  ? 0xffffffffc00d8000
[    1.747334]  __pci_register_driver+0x54/0x60
[    1.747562]  AVM_init+0x36/0x1000 [avmfritz]
[    1.747791]  do_one_initcall+0x48/0x1d0
[    1.747997]  ? __cond_resched+0x19/0x30
[    1.748206]  ? kmem_cache_alloc_trace+0x390/0x440
[    1.748458]  ? do_init_module+0x28/0x250
[    1.748669]  do_init_module+0x62/0x250
[    1.748870]  load_module+0x23ee/0x26a0
[    1.749073]  __do_sys_finit_module+0xc2/0x120
[    1.749307]  ? __do_sys_finit_module+0xc2/0x120
[    1.749549]  __x64_sys_finit_module+0x1a/0x20
[    1.749782]  do_syscall_64+0x38/0x90
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>