cmd/checker: handle unconfigured language
diff --git a/cmd/checker/checker.go b/cmd/checker/checker.go
index 1f6557b..29892af 100644
--- a/cmd/checker/checker.go
+++ b/cmd/checker/checker.go
@@ -141,7 +141,11 @@
 	}
 	req := gerritfmt.FormatRequest{}
 	for n, f := range ch.Files {
-		if !gerritfmt.Formatters[language].Regex.MatchString(n) {
+		cfg := gerritfmt.Formatters[language]
+		if cfg == nil {
+			return nil, fmt.Errorf("language %q not configured")
+		}
+		if !cfg.Regex.MatchString(n) {
 			continue
 		}