Merge pull request #5683 from TheAlphaNerd/validate-before-using-path

fix: don't use path.extname with undefined value

lib/cli-engine.js

@@ -140,13 +140,14 @@ function processText(text, configHelper, filename, fix, allowInlineConfig) {
         config,
         messages,
         stats,
-        fileExtension = path.extname(filename),
+        fileExtension,
         processor,
         loadedPlugins,
         fixedResult;
 
     if (filename) {
         filePath = path.resolve(filename);
+        fileExtension = path.extname(filename);
     }
 
     filename = filename || "<text>";