x86: avoid early crash in disable_local_APIC()

E.g. when called due to an early panic.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/apic.c b/arch/x86/kernel/apic.c
index c3dd64f..38d6aab 100644
--- a/arch/x86/kernel/apic.c
+++ b/arch/x86/kernel/apic.c
@@ -895,6 +895,10 @@
 {
 	unsigned int value;
 
+	/* APIC hasn't been mapped yet */
+	if (!apic_phys)
+		return;
+
 	clear_local_APIC();
 
 	/*