x86/tsc: Prevent NULL pointer deref in calibrate_delay_is_known()

The topology_core_cpumask is used to find a neighbour cpu in
calibrate_delay_is_known(). It might not be allocated at the first invocation
of that function on the boot cpu, when CONFIG_CPUMASK_OFFSTACK is set.

The mask is allocated later in native_smp_prepare_cpus. As a consequence the
underlying find_next_bit() call dereferences a NULL pointer.

Add a proper check to prevent this.

Fixes: c25323c073 "x86/tsc: Use topology functions"
Reported-and-tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1603180843270.3978@nanos
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/tsc.c

@@ -1306,11 +1306,15 @@ void __init tsc_init(void)
 unsigned long calibrate_delay_is_known(void)
 {
 	int sibling, cpu = smp_processor_id();
+	struct cpumask *mask = topology_core_cpumask(cpu);
 
 	if (!tsc_disabled && !cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC))
 		return 0;
 
-	sibling = cpumask_any_but(topology_core_cpumask(cpu), cpu);
+	if (!mask)
+		return 0;
+
+	sibling = cpumask_any_but(mask, cpu);
 	if (sibling < nr_cpu_ids)
 		return cpu_data(sibling).loops_per_jiffy;
 	return 0;