i3c: Add NULL pointer check in i3c_master_queue_ibi()

i3c: Add NULL pointer check in i3c_master_queue_ibi()

The I3C master driver may receive an IBI from a target device that has not
been probed yet. In such cases, the master calls `i3c_master_queue_ibi()`
to queue an IBI work task, leading to "Unable to handle kernel read from
unreadable memory" and resulting in a kernel panic.

Typical IBI handling flow:
1. The I3C master scans target devices and probes their respective drivers.
2. The target device driver calls `i3c_device_request_ibi()` to enable IBI
   and assigns `dev->ibi = ibi`.
3. The I3C master receives an IBI from the target device and calls
   `i3c_master_queue_ibi()` to queue the target device driver’s IBI
   handler task.

However, since target device events are asynchronous to the I3C probe
sequence, step 3 may occur before step 2, causing `dev->ibi` to be `NULL`,
leading to a kernel panic.

Add a NULL pointer check in `i3c_master_queue_ibi()` to prevent accessing
an uninitialized `dev->ibi`, ensuring stability.

Fixes: 3a379bbcea0af ("i3c: Add core I3C infrastructure")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/lkml/Z9gjGYudiYyl3bSe@lizhi-Precision-Tower-5810/
Signed-off-by: Manjunatha Venkatesh <manjunatha.venkatesh@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20250326123047.2797946-1-manjunatha.venkatesh@nxp.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>