fix(collector/pcidevice): Add nil checks to prevent panic

[Tiger1218]

The pcidevice collector panics with a "nil pointer dereference" error
on some systems. This occurs when a PCI device lacks certain attributes
in sysfs, such as 'current_link_speed' or 'current_link_width'.

The collector's Update method attempted to dereference these pointers
without verifying if they were nil, leading to a crash.

This commit introduces checks to verify that the link speed and width
pointers are not nil before they are dereferenced. If an attribute is
missing for a device, its corresponding metric will now be reported as 0,
making the collector more robust and preventing node_exporter from
crashing.