ln: remove panic in result function

found using `clippy::panic_in_result_fn`

Author: xtqqczze

src/uu/ln/locales/en-US.ftl

@@ -26,6 +26,7 @@ ln-help-verbose = print name of each linked file
 ln-error-target-is-not-directory = target {$target} is not a directory
 ln-error-same-file = {$file1} and {$file2} are the same file
 ln-error-missing-destination = missing destination file operand after {$operand}
+ln-error-missing-operand = missing operand
 ln-error-extra-operand = extra operand {$operand}
   Try '{$program} --help' for more information.
 ln-error-could-not-update = Could not update {$target}: {$error}

src/uu/ln/locales/fr-FR.ftl

@@ -27,6 +27,7 @@ ln-help-verbose = afficher le nom de chaque fichier lié
 ln-error-target-is-not-directory = la cible {$target} n'est pas un répertoire
 ln-error-same-file = {$file1} et {$file2} sont le même fichier
 ln-error-missing-destination = opérande de fichier de destination manquant après {$operand}
+ln-error-missing-operand = opérande manquant
 ln-error-extra-operand = opérande supplémentaire {$operand}
   Essayez « {$program} --help » pour plus d'informations.
 ln-error-could-not-update = Impossible de mettre à jour {$target} : {$error}

src/uu/ln/src/ln.rs

@@ -70,6 +70,9 @@ pub enum LnError {
     #[error("{}", translate!("ln-error-missing-destination", "operand" => _0.quote()))]
     MissingDestination(PathBuf),
 
+    #[error("{}", translate!("ln-error-missing-operand"))]
+    MissingOperand,
+
     #[error("{}", translate!("ln-error-extra-operand", "operand" => _0.quote(), "program" => _1.clone()))]
     ExtraOperand(OsString, String),
 
@@ -270,6 +273,10 @@ pub fn uu_app() -> Command {
 ///
 /// This is made public to allow other apps to use `ln` as a library.
 pub fn exec(files: &[PathBuf], settings: &Settings) -> LnResult<()> {
+    if files.is_empty() {
+        return Err(LnError::MissingOperand);
+    }
+
     // Handle cases where we create links in a directory first.
     if let Some(ref target_path) = settings.target_dir {
         // 4th form: a directory is specified by -t.
@@ -298,7 +305,6 @@ pub fn exec(files: &[PathBuf], settings: &Settings) -> LnResult<()> {
             uucore::execution_phrase().to_string(),
         ));
     }
-    assert!(!files.is_empty());
 
     link(&files[0], &files[1], settings)
 }