Fibre Channel collector panics when sysfs statistics counters are missing

Problem

The Fibre Channel collector can panic during a scrape when an FC host does not expose all expected statistics files under /sys/class/fc_host/<host>/statistics.

The procfs library represents unavailable counters as nil pointers, but the collector dereferences them unconditionally.

Panic

node_exporter[1070004]: panic: runtime error: invalid memory address or nil pointer dereference
node_exporter[1070004]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xa2f259]
node_exporter[1070004]: goroutine 89 [running]:
node_exporter[1070004]: github.com/prometheus/node_exporter/collector.(*fibrechannelCollector).Update(0xc00014cf90, 0xc00037fd50)
node_exporter[1070004]:         /Volumes/Home/go/node_exporter/collector/fibrechannel_linux.go:133 +0x479
node_exporter[1070004]: github.com/prometheus/node_exporter/collector.execute({0xc098f3, 0xc}, {0xd34e00, 0xc00014cf90}, 0xc00037fd50, 0xc00003e480)
node_exporter[1070004]:         /Volumes/Home/go/node_exporter/collector/collector.go:160 +0x82
node_exporter[1070004]: github.com/prometheus/node_exporter/collector.NodeCollector.Collect.func1({0xc098f3?, 0x0?}, {0xd34e00?, 0xc00014cf90?})
node_exporter[1070004]:         /Volumes/Home/go/node_exporter/collector/collector.go:151 +0x33
node_exporter[1070004]: created by github.com/prometheus/node_exporter/collector.NodeCollector.Collect in goroutine 60
node_exporter[1070004]:         /Volumes/Home/go/node_exporter/collector/collector.go:150 +0xce

Start in collector/fibrechannel_linux.go at line 133, where the Fibre Channel collector dereferences statistics counters. Check how missing files become nil pointers in the procfs data, then verify the collector handles an FC host with incomplete statistics without panicking during a scrape.