cmdutils: add missing NULL check in parse_options()

Fix ffplay -i FILE, which was recently broken.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

cmdutils.c

@@ -277,7 +277,7 @@ unknown_opt:
                 *po->u.int64_arg = parse_number_or_die(opt, arg, OPT_INT64, INT64_MIN, INT64_MAX);
             } else if (po->flags & OPT_FLOAT) {
                 *po->u.float_arg = parse_number_or_die(opt, arg, OPT_FLOAT, -INFINITY, INFINITY);
-            } else {
+            } else if (po->u.func_arg) {
                 if (po->u.func_arg(opt, arg) < 0) {
                     fprintf(stderr, "%s: failed to set value '%s' for option '%s'\n", argv[0], arg, opt);
                     exit(1);