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.

[yuuki]

I have also encountered this panic error.

In my case, I've got the following stack trace:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xa7e6cd]
goroutine 56 [running]:
github.com/prometheus/node_exporter/collector.(*pcideviceCollector).Update(0xc0002ec640, 0xc0003ea230)
        /home/tsubouchi/node_exporter/collector/pcidevice_linux.go:129 +0x88d
github.com/prometheus/node_exporter/collector.execute({0xcbdf9e, 0x9}, {0xdf17a0, 0xc0002ec640}, 0xc0003ea230, 0xc0001a8470)
        /home/tsubouchi/node_exporter/collector/collector.go:160 +0x85
github.com/prometheus/node_exporter/collector.NodeCollector.Collect.func1({0xcbdf9e?, 0x0?}, {0xdf17a0?, 0xc0002ec640?})
        /home/tsubouchi/node_exporter/collector/collector.go:151 +0x33
created by github.com/prometheus/node_exporter/collector.NodeCollector.Collect in goroutine 43
        /home/tsubouchi/node_exporter/collector/collector.go:150 +0xce

The issue occurs in my environment where current_link_speed is set to Unknown:

$ cat /sys/bus/pci/devices/0000:00:00.0/current_link_speed
Unknown

This PR's fix addresses exactly what I'm experiencing.

[SuperQ]

This will emit a 0 for metrics we don't know the value for.

I think we should do something like this:

Suggested change

	maxLinkSpeedTS = float64(int64(*device.MaxLinkSpeed * 1e9))
	ch <- c.descs[0].mustNewConstMetric(maxLinkSpeedTS, device.Location.Strings()...)

[SuperQ]

This needs a rebase.

[SuperQ]

Needs rebase

[Tiger1218]

Thanks for the review @SuperQ.

It looks like PR #3425 was merged and also fixed the nil pointer panic this PR was intended to address.

Since the issue is now resolved in main, this PR is no longer needed. Closing this.

[SuperQ]

I'm also cleaning up this collector in #3448.

@Tiger1218 Would you mind taking a look at that to make sure I don't reintroduce bugs? 😹