Check Zed.next_error index value is within the string length

This fixes Zed.Out_of_bound errors

src/lib/uTop.ml

@@ -849,23 +849,29 @@ let prompt_comment = ref (S.const [| |])
 module Private = struct
   let fix_string str =
     let len = String.length str in
-    let ofs, _, _ = Zed_utf8.next_error str 0 in
-    if ofs = len then
+    if len = 0 then
       str
-    else begin
-      let buf = Buffer.create (len + 128) in
-      if ofs > 0 then Buffer.add_substring buf str 0 ofs;
-      let rec loop ofs =
-        Zed_utf8.add buf (Uchar.of_char str.[ofs]);
-        let ofs1 = ofs + 1 in
-        let ofs2, _, _ = Zed_utf8.next_error str ofs1 in
-        if ofs1 < ofs2 then
-          Buffer.add_substring buf str ofs1 (ofs2 - ofs1);
-        if ofs2 < len then
-          loop ofs2
-        else
-          Buffer.contents buf
-      in
-      loop ofs
-    end
+    else
+      let ofs, _, _ = Zed_utf8.next_error str 0 in
+      if ofs = len then
+        str
+      else begin
+        let buf = Buffer.create (len + 128) in
+        if ofs > 0 then Buffer.add_substring buf str 0 ofs;
+        let rec loop ofs =
+          Zed_utf8.add buf (Uchar.of_char str.[ofs]);
+          let ofs1 = ofs + 1 in
+          if ofs1 < len then
+            let ofs2, _, _ = Zed_utf8.next_error str ofs1 in
+            if ofs1 < ofs2 then
+              Buffer.add_substring buf str ofs1 (ofs2 - ofs1);
+            if ofs2 < len then
+              loop ofs2
+            else
+              Buffer.contents buf
+          else
+            Buffer.contents buf
+        in
+        loop ofs
+      end
 end

test/test_lib.ml

@@ -7,6 +7,7 @@ let test_fix_string =
   in
   ( "fix_string"
   , [ test ~name:"small" "x" ~expected:"x"
+    ; test ~name:"empty" "" ~expected:""
     ]
   )