x86, ioapic: Throw BUG instead of NULL dereference

Instead of plain NULL deref we better throw error
message with a backtrace. Actually we need more
gracious error handling here. Meanwhile leave it
as is.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: yinghai@kernel.org
LKML-Reference: <20090801075435.769301745@openvz.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>