harded against attempts to build empty truetype fonts

cpdfaddtext.ml

@@ -437,6 +437,7 @@ let
          iter (iter (fun x -> Hashtbl.replace used x ())) codepoints)
     pages
     (map (fun x -> List.nth ps (x - 1)) pages);
+  if Hashtbl.length used = 0 then pdf else (* Avoid trying to build truetype font with no used set. *)
   let realfontname = ref fontname in
   let font, fontpack =
     match cpdffont with

cpdfembed.ml

@@ -89,9 +89,10 @@ let make_fontpack_hashtable fs =
   table
 
 let embed_truetype pdf ~fontfile ~fontname ~codepoints ~encoding =
-  let fs = Cpdftruetype.parse ~subset:codepoints fontfile encoding in
-  let subsets_and_their_fonts = map (make_single_font ~fontname ~encoding pdf) fs in
-    (map snd subsets_and_their_fonts, make_fontpack_hashtable subsets_and_their_fonts)
+  if codepoints = [] then ([], null_hash ()) else (* Can't call Cpdftruetype.parse with empty codepoint set. *)
+    let fs = Cpdftruetype.parse ~subset:codepoints fontfile encoding in
+    let subsets_and_their_fonts = map (make_single_font ~fontname ~encoding pdf) fs in
+      (map snd subsets_and_their_fonts, make_fontpack_hashtable subsets_and_their_fonts)
 
 let rec collate_runs cfn a = function
   | [] -> rev (map rev a)

cpdftruetype.ml

@@ -605,6 +605,7 @@ let parse ~subset data encoding =
               let subset_1, subsets_2 = find_main encoding subset in
               let flags_1 = calculate_flags false italicangle in
               let flags_2 = calculate_flags true italicangle in
+              if subset_1 = [] then raise (Pdf.PDFError "Cpdftruetype: Refusing to create font with no characters") else
               let firstchar_1, lastchar_1 = extremes (sort compare subset_1) in
               let firstchars_2, lastchars_2 = split (map (fun subset -> (33, length subset + 33 - 1)) subsets_2) in
               let numOfLongHorMetrics =