init: silence bogus td == NULL warning

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/init.c b/init.c
index 21779c7..8ed1de6 100644
--- a/init.c
+++ b/init.c
@@ -1907,6 +1907,10 @@
 		case FIO_GETOPT_IOENGINE: {
 			const char *opt = l_opts[lidx].name;
 			char *val = optarg;
+
+			if (!td)
+				break;
+
 			ret = fio_cmd_ioengine_option_parse(td, opt, val);
 			break;
 		}