MacOS: create_cstr_from_cfstring uses safe conversion - use CFStringGetCString

During testing with gdk-pixbuf I noticed failures during content type
to mime conversion. The root reason was the unsafe conversion used
in create_cstr_from_cfstring. The problem was addressed in commit
c60226e0a1 but that was reverted. I noticed the commit only
when I had fixed the problem. In addition I added a test to check
the content type to mime conversion on MacOS. This problem is
discussed in Bug #788936.

See: https://bugzilla.gnome.org/show_bug.cgi?id=788936

gio/gosxappinfo.c

@@ -171,19 +171,26 @@ create_cfstring_from_cstr (const gchar *cstr)
   return CFStringCreateWithCString (NULL, cstr, kCFStringEncodingUTF8);
 }
 
+#ifdef G_ENABLE_DEBUG
 static gchar *
 create_cstr_from_cfstring (CFStringRef str)
 {
-  const gchar *cstr;
+  g_return_val_if_fail (str != NULL, NULL);
 
-  if (str == NULL)
-    return NULL;
-
-  cstr = CFStringGetCStringPtr (str, kCFStringEncodingUTF8);
-  CFRelease (str);
-
-  return g_strdup (cstr);
+  CFIndex length = CFStringGetLength (str);
+  CFIndex maxlen = CFStringGetMaximumSizeForEncoding (length, kCFStringEncodingUTF8);
+  gchar *buffer = g_malloc (maxlen + 1);
+  Boolean success = CFStringGetCString (str, (char *) buffer, maxlen,
+                                        kCFStringEncodingUTF8);
+  if (success)
+    return buffer;
+  else
+    {
+      g_free (buffer);
+      return NULL;
+    }
 }
+#endif
 
 static char *
 url_escape_hostname (const char *url)