firewire: ohci: fix NULL pointer dereference in ar_context_release

firewire: ohci: fix NULL pointer dereference in ar_context_release

During the error handling path of the driver's probe function, a NULL
pointer dereference can occur in ar_context_release().

When pci_probe() fails early (e.g., if pcim_enable_device() or MMIO mapping
fails), the devres cleanup mechanism invokes release_ohci(). This function
unconditionally calls ar_context_release() to clean up the asynchronous
receive contexts. However, if ar_context_init() was not yet called,
ctx->ohci remains NULL (as the fw_ohci structure is zero-initialized by
devres_alloc()).

ar_context_release() immediately dereferences ctx->ohci to get the dev
pointer before checking if the context was actually initialized, leading to
a crash:

Oops: general protection fault, probably for non-canonical address
0xdffffc0000000001: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
RIP: 0010:ar_context_release+0x3f/0x380 drivers/firewire/ohci.c:543
Call Trace:
 release_ohci+0x3f/0x60 drivers/firewire/ohci.c:3567
 release_nodes drivers/base/devres.c:546 [inline]
 devres_release_all+0x1a8/0x260 drivers/base/devres.c:576
 device_unbind_cleanup drivers/base/dd.c:597 [inline]
 really_probe+0x451/0xae0 drivers/base/dd.c:772

To fix this, move the assignment of the dev pointer after the !ctx->buffer
check. If ctx->buffer is NULL, it indicates that the context was never
successfully initialized and there is nothing to release, safely avoiding
the dereference of the uninitialized ctx->ohci pointer.

Fixes: 5716e58aecdd ("firewire: ohci: release buffer for AR req/resp contexts when managed resource is released")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+d30aad27833a559defab@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d30aad27833a559defab
Link: https://syzkaller.appspot.com/ai_job?id=10a18617-7893-42dd-bf1c-cd49e19e95d9
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
Link: https://lore.kernel.org/r/90c5db71-dd1f-4d46-b9d3-2f1046cbd5ea@mail.kernel.org
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>