Check for input file before trying to use

src/index.js

@@ -18,14 +18,15 @@ program
   .parse(process.argv)
 
 let inputFile = program.args[0]
-let outputFile =
-  program.args[1] || `${splitFileName(program.args[0])[0]}-output.css`
 
 if (!inputFile) {
   console.error('No input file given!')
   process.exit(1)
 }
 
+let outputFile =
+  program.args[1] || `${splitFileName(program.args[0])[0]}-output.css`
+
 fs.readFile(path.join(program.config), (err, config) => {
   if (err) {
     console.error(`config file ${program.config} does not exist`)