fix CCString: clear buffer after use in sprintf

fix #487

src/core/CCFormat.ml

@@ -398,7 +398,10 @@ let sprintf_ c format =
   Format.kfprintf
     (fun _fmt ->
       Format.pp_print_flush fmt ();
-      Buffer.contents buf)
+      let s = Buffer.contents buf in
+      Buffer.reset buf;
+      (* in case of reuse *)
+      s)
     fmt format
 
 let with_color_ksf ~f s fmt =

tests/core/t_string.ml

@@ -410,4 +410,7 @@ q
 ;;
 
 t @@ fun () -> "ab" < "abc";;
-t @@ fun () -> "123" < "14"
+t @@ fun () -> "123" < "14";;
+
+eq ~printer:Fun.id "a,b,c,d,e,f,g,"
+  (flat_map (CCFormat.sprintf "%c,") "abcdefg")

tests/pvec/t_pvec.ml

@@ -266,13 +266,15 @@ module Op = struct
                      list_small gen_x >|= fun l ->
                      Append l, size + List.length l );
                  ];
-                 (if size < 10_000 then [
+                 (if size < 10_000 then
+                    [
                       (* flat map can explode, only do it if list isn't too big *)
                       ( 1,
                         list_size (0 -- 5) gen_x >|= fun l ->
                         Flat_map l, size * (1 + List.length l) );
-                 ] else [])
+                    ]
-                 ;
+                  else
+                    []);
                ]
         in