sysfs: Fix crash on empty group attributes array

sysfs: Fix crash on empty group attributes array

It turns out that arch/x86/events/intel/core.c makes use of "empty"
attributes.

	static struct attribute *empty_attrs;

	__init int intel_pmu_init(void)
	{
	        struct attribute **extra_skl_attr = &empty_attrs;
	        struct attribute **extra_attr = &empty_attrs;
	        struct attribute **td_attr    = &empty_attrs;
	        struct attribute **mem_attr   = &empty_attrs;
	        struct attribute **tsx_attr   = &empty_attrs;
		...

That breaks the assumption __first_visible() that expects that if
grp->attrs is set then grp->attrs[0] must also be set and results in
backtraces like:

    BUG: kernel NULL pointer dereference, address: 00rnel mode
    #PF: error_code(0x0000) - not-present ] PREEMPT SMP NOPTI
    CPU: 1 PID: 1 Comm: swapper/IP: 0010:exra_is_visible+0x14/0x20
     ? exc_page_fault+0x68/0x190
     internal_create_groups+0x42/0xa0
     pmu_dev_alloc+0xc0/0xe0
     perf_event_sysfs_init+0x580000000000 ]---
    RIP: 0010:exra_is_visible+0x14/0

Check for non-empty attributes array before calling is_visible().

Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Closes: https://github.com/thesofproject/linux/pull/4799#issuecomment-1958537212
Fixes: 70317fd24b41 ("sysfs: Introduce a mechanism to hide static attribute_groups")
Cc: Marc Herbert <marc.herbert@intel.com>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Marc Herbert <marc.herbert@intel.com>
Link: https://lore.kernel.org/r/170863445442.1479840.1818801787239831650.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>