drivers/cpufreq/pcc-cpufreq.c: avoid NULL pointer dereference

per_cpu(processors, n) can be NULL, resulting in:

Loading CPUFreq modules[ 437.661360] BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffffa0434314>] pcc_cpufreq_cpu_init+0x74/0x220 [pcc_cpufreq]

It's better to avoid the oops by failing the driver, and allowing the
system to boot.

Signed-off-by: Naga Chumbalkar <[e-mail]>
Cc: Dave Jones <[e-mail]>
Cc: Len Brown <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/cpufreq/pcc-cpufreq.c

···
 	pr = per_cpu(processors, cpu);
 	pcc_cpu_data = per_cpu_ptr(pcc_cpu_info, cpu);
 
+	if (!pr)
+		return -ENODEV;
+
 	status = acpi_evaluate_object(pr->handle, "PCCP", NULL, &buffer);
 	if (ACPI_FAILURE(status))
 		return -ENODEV;