Check that the argument to Digest.to_hex has the correct length (#763)

* Check that the argument to Digest.to_hex has the correct length and improve documentation for Digest.to_hex

stdlib/digest.ml

@@ -50,6 +50,7 @@ let char_hex n =
   Char.unsafe_chr (n + if n < 10 then Char.code '0' else (Char.code 'a' - 10))
 
 let to_hex d =
+  if String.length d <> 16 then invalid_arg "Digest.to_hex";
   let result = Bytes.create 32 in
   for i = 0 to 15 do
     let x = Char.code d.[i] in
@@ -59,7 +60,7 @@ let to_hex d =
   Bytes.unsafe_to_string result
 
 let from_hex s =
-  if String.length s <> 32 then raise (Invalid_argument "Digest.from_hex");
+  if String.length s <> 32 then invalid_arg "Digest.from_hex";
   let digit c =
     match c with
     | '0'..'9' -> Char.code c - Char.code '0'

stdlib/digest.mli

@@ -73,7 +73,9 @@ val input : in_channel -> t
 (** Read a digest from the given input channel. *)
 
 val to_hex : t -> string
-(** Return the printable hexadecimal representation of the given digest. *)
+(** Return the printable hexadecimal representation of the given digest.
+    Raise [Invalid_argument] if the argument is not exactly 16 bytes.
+ *)
 
 val from_hex : string -> t
 (** Convert a hexadecimal representation back into the corresponding digest.