printf: add a test for %x with invalid value

tests/by-util/test_printf.rs

@@ -258,6 +258,14 @@ fn sub_num_hex_upper() {
         .stdout_only("thirty in hex is 1E");
 }
 
+#[test]
+fn sub_num_hex_non_numerical() {
+    new_ucmd!()
+        .args(&["parameters need to be numbers %X", "%194"])
+        .fails()
+        .code_is(1);
+}
+
 #[test]
 fn sub_num_float() {
     new_ucmd!()