Eliminate an unreachable codepath from String::from_utf8_lossy

Utf8Lossy's Iterator implementation ensures that only the final chunk
has an empty slice for broken. Thus the only way the first chunk could
have an empty broken is if it is the final chunk, i.e. there is only one
chunk total. And the only way that there could be one chunk total is if
the whole input is valid utf8 and non-empty. That condition has already
been handled by an early return, so at the point that the first
REPLACEMENT is being pushed, it's impossible for first_broken to be
empty.

library/alloc/src/string.rs

@@ -558,13 +558,13 @@ impl String {
     pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str> {
         let mut iter = lossy::Utf8Lossy::from_bytes(v).chunks();
 
-        let (first_valid, first_broken) = if let Some(chunk) = iter.next() {
+        let first_valid = if let Some(chunk) = iter.next() {
             let lossy::Utf8LossyChunk { valid, broken } = chunk;
-            if valid.len() == v.len() {
-                debug_assert!(broken.is_empty());
+            if broken.is_empty() {
+                debug_assert_eq!(valid.len(), v.len());
                 return Cow::Borrowed(valid);
             }
-            (valid, broken)
+            valid
         } else {
             return Cow::Borrowed("");
         };
@@ -573,9 +573,7 @@ impl String {
 
         let mut res = String::with_capacity(v.len());
         res.push_str(first_valid);
-        if !first_broken.is_empty() {
-            res.push_str(REPLACEMENT);
-        }
+        res.push_str(REPLACEMENT);
 
         for lossy::Utf8LossyChunk { valid, broken } in iter {
             res.push_str(valid);