ar5523: Fix null-ptr-deref with unexpected WDCMSG_TARGET_START reply

Unexpected WDCMSG_TARGET_START replay can lead to null-ptr-deref
when ar->tx_cmd->odata is NULL. The patch adds a null check to
prevent such case.

KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
 ar5523_cmd+0x46a/0x581 [ar5523]
 ar5523_probe.cold+0x1b7/0x18da [ar5523]
 ? ar5523_cmd_rx_cb+0x7a0/0x7a0 [ar5523]
 ? __pm_runtime_set_status+0x54a/0x8f0
 ? _raw_spin_trylock_bh+0x120/0x120
 ? pm_runtime_barrier+0x220/0x220
 ? __pm_runtime_resume+0xb1/0xf0
 usb_probe_interface+0x25b/0x710
 really_probe+0x209/0x5d0
 driver_probe_device+0xc6/0x1b0
 device_driver_attach+0xe2/0x120

I found the bug using a custome USBFuzz port. It's a research work
to fuzz USB stack/drivers. I modified it to fuzz ath9k driver only,
providing hand-crafted usb descriptors to QEMU.

After fixing the code (fourth byte in usb packet) to WDCMSG_TARGET_START,
I got the null-ptr-deref bug. I believe the bug is triggerable whenever
cmd->odata is NULL. After patching, I tested with the same input and no
longer see the KASAN report.

This was NOT tested on a real device.

Signed-off-by: Zekun Shen <bruceshenzk@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/YXsmPQ3awHFLuAj2@10-18-43-117.dynapool.wireless.nyu.edu