Fixed empty backslash

Signed-off-by: Hanif Bin Ariffin <hanif.ariffin.4326@gmail.com>

src/uu/tr/src/operation.rs

@@ -235,9 +235,20 @@ impl Sequence {
             (
                 l,
                 if let Some(input) = a.strip_prefix('\\') {
-                    char::from_u32(u32::from_str_radix(&input, 8).unwrap()).unwrap()
+                    if input.is_empty() {
+                        '\\'
                     } else {
-                    input.chars().next().unwrap()
+                        char::from_u32(
+                            u32::from_str_radix(&input, 8)
+                                .expect("We only matched against 0-7 so it should not fail"),
+                        )
+                        .expect("Cannot convert octal value to character")
+                    }
+                } else {
+                    input
+                        .chars()
+                        .next()
+                        .expect("We recognized a character so this should not fail")
                 },
             )
         })

src/uu/tr/src/tr.rs

@@ -112,6 +112,14 @@ pub fn uumain(args: impl uucore::Args) -> i32 {
             return 1;
         }
     };
+
+    if set2.len() == 1 && set2[0] == '\\' {
+        show_error!(
+            "{}",
+            "warning: an unescaped backslash at end of string is not portable"
+        );
+    }
+
     if delete_flag {
         if squeeze_flag {
             let mut delete_buffer = vec![];