[CM] Snd: save-mix segfault

[Tito Latini]

Hi, if a mix doesn't exist, Snd crashes after save-mix.

Example I

(define mix7 (integer->mix 7))
=> #<mix 7>

(mix? mix7)
=> #f

(save-mix mix7 "bye.snd")
-- segfault

Example II

- open-sound
- add a mix
- (define mix0 (caaar (mixes)))
- save-sound
- (save-mix mix0 "crash.snd")

The null pointer (returned by md_from_id) is in save_mix (snd-mix.c:3681),
called only by copy_mix and g_save_mix. copy_mix is safe because

  md = md_from_id(id);
  if (!md) return(-1);

save_mix returns a io_error_t and there is not a value suitable
for a mix, so the attached patch fixes the bug in g_save_mix.
-------------- next part --------------
diff -ur snd-15~/snd-mix.c snd-15/snd-mix.c
--- snd-15~/snd-mix.c	2015-04-02 23:49:25.463887691 +0200
+++ snd-15/snd-mix.c	2015-04-02 23:49:37.209740026 +0200
@@ -3788,12 +3788,15 @@
 static Xen g_save_mix(Xen m, Xen file)
 {
   #define H_save_mix "(" S_save_mix " mix filename) saves mix's samples in the file 'filename'"
-
+  int id;
   Xen_check_type(xen_is_mix(m), m, 1, S_save_mix, "a mix");
-  Xen_check_type(Xen_is_string(file), file, 2, S_save_mix, "a filename");
-
-  save_mix(Xen_mix_to_C_int(m), Xen_string_to_C_string(file), MUS_NEXT, MUS_OUT_SAMPLE_TYPE);
-  return(m);
+  id = Xen_mix_to_C_int(m);
+  if (mix_is_active(id)) {
+    Xen_check_type(Xen_is_string(file), file, 2, S_save_mix, "a filename");
+    save_mix(id, Xen_string_to_C_string(file), MUS_NEXT, MUS_OUT_SAMPLE_TYPE);
+    return(m);
+  }
+  return(snd_no_such_mix_error(S_save_mix, m));
 }