MIPS: SGI-IP30: Check for valid pointer before using it

Fix issue detected by Smatch:

    ./arch/mips/sgi-ip30/ip30-irq.c:236 heart_domain_free()
     warn: variable dereferenced before check 'irqd' (see line 235)

Fixes: 7505576d

 ("MIPS: add support for SGI Octane (IP30)")
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: <stable@vger.kernel.org> # v5.5+