While investigating googleapis/api-linter#908, added the following log statements:

```go
func (l *linker) checkForUnusedImports(filename string) {
	log.Println("checkForUnusedImports", filename)
	r := l.files[filename]
	usedImports := l.usedImports[r.fd]
	node := r.nodes[r.fd]
	fileNode, _ := node.(*ast.FileNode)
	for i, dep := range r.fd.Dependency {
		log.Println("checkForUnusedImports", i, dep)

		if _, ok := usedImports[dep]; !ok {
			isPublic := false
			// it's fine if it's a public import
			for _, j := range r.fd.PublicDependency {
				if i == int(j) {
					isPublic = true
					break
				}
			}
			if isPublic {
				break
			}
			var pos *SourcePos
			if fileNode != nil {
				log.Printf("checkForUnusedImports fileNode decls: %+v (%T)", fileNode.Decls, fileNode.Decls)

				for i, decl := range fileNode.Decls {
					log.Println("checkForUnusedImports decls", i, decl)

					imp, ok := decl.(*ast.ImportNode)
					if !ok {
						continue
					}
					if imp.Name.AsString() == dep {
						pos = imp.Start()
					}
				}
			}
			if pos == nil {
				pos = ast.UnknownPos(r.fd.GetName())
			}
			log.Println("checkForUnusedImports err warn", r.errs, pos, dep)

			r.errs.warn(pos, errUnusedImport(dep))
		}
	}
}
```

And observed the following:

```
2021/12/15 05:33:29 checkForUnusedImports rosetta/rosetta/pluto/v1/service.proto
2021/12/15 05:33:29 checkForUnusedImports 0 google/protobuf/timestamp.proto
2021/12/15 05:33:29 checkForUnusedImports err warn <nil> rosetta/rosetta/pluto/v1/service.proto google/protobuf/timestamp.proto
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x130ea32]

goroutine 1 [running]:
github.com/jhump/protoreflect/desc/protoparse.(*errorHandler).warn(...)
	external/com_github_jhump_protoreflect/desc/protoparse/errors.go:80
github.com/jhump/protoreflect/desc/protoparse.(*linker).checkForUnusedImports(0xc000094c30, 0xc00002a751, 0x26)
	external/com_github_jhump_protoreflect/desc/protoparse/linker.go:931 +0x472
github.com/jhump/protoreflect/desc/protoparse.Parser.ParseFiles(0xc0003ed6c0, 0x1, 0x1, 0x0, 0xc0004cc790, 0x0, 0x0, 0x1, 0xc000129080, 0x0, ...)
	external/com_github_jhump_protoreflect/desc/protoparse/parser.go:238 +0x373
main.(*cli).lint(0xc000143740, 0xc00040faa0, 0x1b47370, 0x2, 0x2, 0x0, 0x0)
	external/com_github_googleapis_api_linter/cmd/api-linter/cli.go:161 +0x427
main.runCLI(0xc000142010, 0xb, 0xb, 0x0, 0x0)
	external/com_github_googleapis_api_linter/cmd/api-linter/main.go:44 +0x85
main.main()
	external/com_github_googleapis_api_linter/cmd/api-linter/main.go:37 +0x76
```

So this panic occurred because the `errs` struct on the `parseResult` was nil.  This led me to the current diff, which recovers the panic in my case.