serial: 8250_port: Check IRQ data before use

In case the leaf driver wants to use IRQ polling (irq = 0) and
IIR register shows that an interrupt happened in the 8250 hardware
the IRQ data can be NULL. In such a case we need to skip the wake
event as we came to this path from the timer interrupt and quite
likely system is already awake.

Without this fix we have got an Oops:

    serial8250: ttyS0 at I/O 0x3f8 (irq = 0, base_baud = 115200) is a 16550A
    ...
    BUG: kernel NULL pointer dereference, address: 0000000000000010
    RIP: 0010:serial8250_handle_irq+0x7c/0x240
    Call Trace:
     ? serial8250_handle_irq+0x7c/0x240
     ? __pfx_serial8250_timeout+0x10/0x10

Fixes: 0ba9e3a1

 ("serial: 8250: Add missing wakeup event reporting")
Cc: stable <stable@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20230831222555.614426-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>