pr: reject a non-positive expand-tab width (-e0)

`pr -e0` on input containing a tab aborted with a remainder-by-zero panic
(`chunk.len() % expand_options.width` in `apply_expand_tab`). The
`-e[char][width]` parser accepted a width of `0` — and a negative width via
e.g. `-eX-5` — without validation, so the zero/negative reached the chunk
arithmetic.

Reject `width <= 0` while parsing `-e`, emitting the same invalid-argument
error GNU produces (`pr: '-e' extra characters or invalid number in the
argument: '0'`, exit 1) instead of crashing.

tests/by-util/test_pr.rs

@@ -889,3 +889,16 @@ fn test_zero_columns_shortcut() {
         .fails_with_code(1)
         .stderr_contains("pr: invalid --column argument '0'");
 }
+
+#[test]
+fn test_zero_expand_tab_width() {
+    let expected = "pr: '-e' extra characters or invalid number in the argument: ‘0’\nTry 'pr --help' for more information.\n";
+    new_ucmd!()
+        .arg("-e0")
+        .fails_with_code(1)
+        .stderr_only(expected);
+    new_ucmd!()
+        .arg("-eX0")
+        .fails_with_code(1)
+        .stderr_only(expected);
+}