Fix clippy needless_late_init in txt_helpers (#2332)

Master fails `cargo clippy -- -D warnings` on lib_ccxr, so format_rust
is red on every open pull request regardless of what it changes. Authors
have been fixing it inside unrelated PRs to get their CI green, which
couples a one-line lint fix to whatever else they are working on.

Assign c_len from an if-expression instead of declaring it and filling
it in on each branch. Behaviour is identical.

src/rust/lib_ccxr/src/encoder/txt_helpers.rs

@@ -42,21 +42,21 @@ fn change_utf8_encoding(dest: &mut [u8], src: &[u8], len: i32, out_enc: Encoding
 
     while src_idx < max {
         let c = src[src_idx];
-        let c_len: usize;
-
-        if c < 0x80 {
-            c_len = 1;
+        // Leading-byte width. A byte that matches none of the UTF-8 lead
+        // patterns is consumed one at a time so the loop always advances.
+        let c_len: usize = if c < 0x80 {
+            1
         } else if (c & 0x20) == 0 {
-            c_len = 2;
+            2
         } else if (c & 0x10) == 0 {
-            c_len = 3;
+            3
         } else if (c & 0x08) == 0 {
-            c_len = 4;
+            4
         } else if (c & 0x04) == 0 {
-            c_len = 5;
+            5
         } else {
-            c_len = 1;
-        }
+            1
+        };
 
         match out_enc {
             Encoding::UTF8 => {