PCI/AER: Avoid NULL pointer dereference in aer_ratelimit()

In the Linux kernel, the following vulnerability has been resolved:

PCI/AER: Avoid NULL pointer dereference in aer_ratelimit()

When platform firmware supplies error information to the OS, e.g., via the
ACPI APEI GHES mechanism, it may identify an error source device that
doesn't advertise an AER Capability and therefore dev->aer_info, which
contains AER stats and ratelimiting data, is NULL.

pci_dev_aer_stats_incr() already checks dev->aer_info for NULL, but
aer_ratelimit() did not, leading to NULL pointer dereferences like this one
from the URL below:

{1}[Hardware Error]: Hardware error from APEI Generic Hardware Error Source: 0
{1}[Hardware Error]: event severity: corrected
{1}[Hardware Error]: device_id: 0000:00:00.0
{1}[Hardware Error]: vendor_id: 0x8086, device_id: 0x2020
{1}[Hardware Error]: aer_cor_status: 0x00001000, aer_cor_mask: 0x00002000
BUG: kernel NULL pointer dereference, address: 0000000000000264
RIP: 0010:___ratelimit+0xc/0x1b0
pci_print_aer+0x141/0x360
aer_recover_work_func+0xb5/0x130

[8086:2020] is an Intel "Sky Lake-E DMI3 Registers" device that claims to
be a Root Port but does not advertise an AER Capability.

Add a NULL check in aer_ratelimit() to avoid the NULL pointer dereference.
Note that this also prevents ratelimiting these events from GHES.

[bhelgaas: add crash details to commit log]