target-i386: fix "info lapic" segfault on isapc

Start QEMU with
"qemu-system-x86_64 -nographic -M isapc -serial none-monitor stdio"
and enter "info lapic" at the monitor prompt ⇒
Segmentation fault

Signed-off-by: Tejaswini Poluri <tejaswinipoluri3@gmail.com>
Message-Id: <1490685583-16987-1-git-send-email-tejaswinipoluri3@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/helper.c

@@ -326,6 +326,10 @@ void x86_cpu_dump_local_apic_state(CPUState *cs, FILE *f,
 {
     X86CPU *cpu = X86_CPU(cs);
     APICCommonState *s = APIC_COMMON(cpu->apic_state);
+    if (!s) {
+        cpu_fprintf(f, "local apic state not available\n");
+        return;
+    }
     uint32_t *lvt = s->lvt;
 
     cpu_fprintf(f, "dumping local APIC state for CPU %-2u\n\n",