numfmt: do not panic on a multibyte whitespace field separator

write_formatted_with_whitespace normalized the delimiter before the
second and subsequent selected fields by writing a single space and
dropping the first byte of the leading whitespace run with `&prefix[1..]`.
That assumed an ASCII separator: WhitespaceSplitter splits on Unicode
`char::is_whitespace`, so a multibyte separator (e.g. U+3000 IDEOGRAPHIC
SPACE) made `&prefix[1..]` slice inside the character and abort with a
char-boundary panic.

Drop the first character instead of the first byte, using its UTF-8
length, so the slice always lands on a char boundary.

Author: leeewee

src/uu/numfmt/src/format.rs

@@ -845,7 +845,7 @@ pub fn write_formatted_with_whitespace<W: std::io::Write + ?Sized>(
             // add delimiter before second and subsequent fields
             let prefix = if n > 1 {
                 writer.write_all(b" ").unwrap();
-                &prefix[1..]
+                &prefix[prefix.chars().next().map_or(0, char::len_utf8)..]
             } else {
                 prefix
             };

tests/by-util/test_numfmt.rs

@@ -431,6 +431,16 @@ fn test_format_selected_field() {
         .stdout_only("1K 2000 3K\n");
 }
 
+#[test]
+fn test_field_with_multibyte_whitespace_separator() {
+    // A multibyte Unicode whitespace separator (U+3000) before a selected field
+    // is normalized to a single space rather than sliced mid-character.
+    new_ucmd!()
+        .args(&["--field", "2", "1　2"])
+        .succeeds()
+        .stdout_only("1 2\n");
+}
+
 #[test]
 fn test_format_selected_fields() {
     new_ucmd!()