ACPI: Add NULL pointer check in acpi_bus_start

If acpi_bus_add does not return a device and it's passed
to acpi_bus_start, bad things will happen:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
IP: [<ffffffff8128402d>] acpi_bus_start+0x14/0x24
...
[<ffffffffa008977a>] acpiphp_bus_add+0xba/0x130 [acpiphp]
[<ffffffffa008aa72>] enable_device+0x132/0x2ff [acpiphp]
[<ffffffffa0089b68>] acpiphp_enable_slot+0xb8/0x130 [acpiphp]
[<ffffffffa0089df7>] handle_hotplug_event_func+0x87/0x190 [acpiphp]

Next patch would make this NULL pointer check obsolete, but
better having one more than one missing...

Signed-off-by: Thomas Renninger <trenn@suse.de>
Acked-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
CC: stable@kernel.org
Signed-off-by: Len Brown <len.brown@intel.com>