i386: handle a negative return value

The Coverity checker noted that bad things might happen if
find_isa_irq_apic() returned -1.

[akpm@osdl.org: add debugging checks]
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Andi Kleen <ak@suse.de>
Acked-by: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
?