New utility function burnwcs().

Just like burnstr(), it memsets a NUL-terminated string to all zeroes
before freeing it. The only difference is that it does it to a string
of wchar_t.

utils/CMakeLists.txt

@@ -9,6 +9,7 @@ add_sources_from_current_dir(utils
   bufchain.c
   buildinfo.c
   burnstr.c
+  burnwcs.c
   cert-expr.c
   chomp.c
   cmdline_get_passwd_input_state_new.c

utils/burnwcs.c

@@ -0,0 +1,18 @@
+/*
+ * 'Burn' a dynamically allocated wide string, in the sense of
+ * destroying it beyond recovery: overwrite it with zeroes and then
+ * free it.
+ */
+
+#include <wchar.h>
+
+#include "defs.h"
+#include "misc.h"
+
+void burnwcs(wchar_t *string)
+{
+    if (string) {
+        smemclr(string, sizeof(*string) * wcslen(string));
+        sfree(string);
+    }
+}