x86/apic: Fix UP boot crash

Commit 31b3c9d7 ("xen/x86: Implement x86_apic_ops") implemented
this:

... without considering that on UP the function pointer might be NULL.

Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Link: http://lkml.kernel.org/n/tip-3pfty0ml4yp62phbkchichh0@git.kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/kernel/setup.c

@@ -1012,7 +1012,8 @@ void __init setup_arch(char **cmdline_p)
 	init_cpu_to_node();
 
 	init_apic_mappings();
-	x86_io_apic_ops.init();
+	if (x86_io_apic_ops.init)
+		x86_io_apic_ops.init();
 
 	kvm_guest_init();