Check Zed.next_error index value is within the string length (#442)

This fixes Zed.Out_of_bound errors

Co-authored-by: Etienne Millon <me@emillon.org>

src/lib/uTop.ml

@@ -849,6 +849,9 @@ let prompt_comment = ref (S.const [| |])
 module Private = struct
   let fix_string str =
     let len = String.length str in
+    if len = 0 then
+      str
+    else
       let ofs, _, _ = Zed_utf8.next_error str 0 in
       if ofs = len then
         str
@@ -858,6 +861,7 @@ module Private = struct
         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);
@@ -865,6 +869,8 @@ module Private = struct
               loop ofs2
             else
               Buffer.contents buf
+          else
+            Buffer.contents buf
         in
         loop ofs
       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:""
     ]
   )