split: correct error message on invalid arg. to -a

Correct the error message displayed on an invalid parameter to the
`--suffix-length` or `-a` command-line option.

src/uu/split/src/split.rs

@@ -233,12 +233,14 @@ struct Settings {
 impl Settings {
     /// Parse a strategy from the command-line arguments.
     fn from(matches: &ArgMatches) -> UResult<Self> {
+        let suffix_length_str = matches.value_of(OPT_SUFFIX_LENGTH).unwrap();
         let result = Self {
-            suffix_length: matches
-                .value_of(OPT_SUFFIX_LENGTH)
-                .unwrap()
-                .parse()
-                .unwrap_or_else(|_| panic!("Invalid number for {}", OPT_SUFFIX_LENGTH)),
+            suffix_length: suffix_length_str.parse().map_err(|_| {
+                USimpleError::new(
+                    1,
+                    format!("invalid suffix length: {}", suffix_length_str.quote()),
+                )
+            })?,
             numeric_suffix: matches.occurrences_of(OPT_NUMERIC_SUFFIXES) > 0,
             additional_suffix: matches.value_of(OPT_ADDITIONAL_SUFFIX).unwrap().to_owned(),
             verbose: matches.occurrences_of("verbose") > 0,

tests/by-util/test_split.rs

@@ -440,3 +440,12 @@ fn test_number() {
     assert_eq!(file_read("xad"), "pqrst");
     assert_eq!(file_read("xae"), "uvwxyz");
 }
+
+#[test]
+fn test_invalid_suffix_length() {
+    new_ucmd!()
+        .args(&["-a", "xyz"])
+        .fails()
+        .no_stdout()
+        .stderr_contains("invalid suffix length: 'xyz'");
+}