Catch NULL td->o.ioengine

This should not happen, except if there's a bug in the
option parsing (or profile, see commit 774a99b5). But
lets be cautious and catch it instead of seg faulting.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/init.c b/init.c
index c44fa67..70b56e3 100644
--- a/init.c
+++ b/init.c
@@ -749,6 +749,10 @@
 	 */
 	if (td->io_ops)
 		return 0;
+	if (!td->o.ioengine) {
+		log_err("fio: internal fault, no IO engine specified\n");
+		return 1;
+	}
 
 	engine = get_engine_name(td->o.ioengine);
 	td->io_ops = load_ioengine(td, engine);