daemon: Allow free_strings (NULL) and free_stringslen (NULL, ..).

Equivalent to free (NULL).

daemon/guestfsd.c

@@ -517,6 +517,9 @@ free_strings (char **argv)
 {
   size_t argc;
 
+  if (!argv)
+    return;
+
   for (argc = 0; argv[argc] != NULL; ++argc)
     free (argv[argc]);
   free (argv);
@@ -527,6 +530,9 @@ free_stringslen (char **argv, size_t len)
 {
   size_t i;
 
+  if (!argv)
+    return;
+
   for (i = 0; i < len; ++i)
     free (argv[i]);
   free (argv);