Check Zed.next_error index value is within the string length

This fixes Zed.Out_of_bound errors

src/lib/uTop_main.ml

@@ -413,25 +413,31 @@ end
 
 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
 
 let render_out_phrase term string =
   if String.length string >= 100 * 1024 then