syzbot reported null pointer dereference in usb_audio_probe.
The problem was in case, when quirk == NULL. It's not an
error condition, so quirk must be checked before dereferencing.

Call Trace:
 usb_probe_interface+0x315/0x7f0 drivers/usb/core/driver.c:396
 really_probe+0x291/0xe60 drivers/base/dd.c:554
 driver_probe_device+0x26b/0x3d0 drivers/base/dd.c:740
 __device_attach_driver+0x1d1/0x290 drivers/base/dd.c:846
 bus_for_each_drv+0x15f/0x1e0 drivers/base/bus.c:431
 __device_attach+0x228/0x4a0 drivers/base/dd.c:914
 bus_probe_device+0x1e4/0x290 drivers/base/bus.c:491
 device_add+0xbdb/0x1db0 drivers/base/core.c:3242
 usb_set_configuration+0x113f/0x1910 drivers/usb/core/message.c:2164
 usb_generic_driver_probe+0xba/0x100 drivers/usb/core/generic.c:238
 usb_probe_device+0xd9/0x2c0 drivers/usb/core/driver.c:293
 really_probe+0x291/0xe60 drivers/base/dd.c:554
 driver_probe_device+0x26b/0x3d0 drivers/base/dd.c:740
 __device_attach_driver+0x1d1/0x290 drivers/base/dd.c:846
 bus_for_each_drv+0x15f/0x1e0 drivers/base/bus.c:431
 __device_attach+0x228/0x4a0 drivers/base/dd.c:914
 bus_probe_device+0x1e4/0x290 drivers/base/bus.c:491
 device_add+0xbdb/0x1db0 drivers/base/core.c:3242
 usb_new_device.cold+0x721/0x1058 drivers/usb/core/hub.c:2555
 hub_port_connect drivers/usb/core/hub.c:5223 [inline]
 hub_port_connect_change drivers/usb/core/hub.c:5363 [inline]
 port_event drivers/usb/core/hub.c:5509 [inline]
 hub_event+0x2357/0x4320 drivers/usb/core/hub.c:5591
 process_one_work+0x98d/0x1600 kernel/workqueue.c:2275
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2421
 kthread+0x3b1/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported-by: syzbot+719da9b149a931f5143f@syzkaller.appspotmail.com
Fixes: 97991108 ("ALSA: usb-audio: Disable USB autosuspend properly in setup_disable_autosuspend()")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/f1ebad6e721412843bd1b12584444c0a63c6b2fb.1615242183.git.paskripkin@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>