IO verification needs to open and grab a reference to a file

Otherwise it can disappear in midair.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index c824d7e..6290ebb 100644
--- a/io_u.c
+++ b/io_u.c
@@ -593,6 +593,8 @@
 		}
 	} while (1);
 
+	assert(io_u->file->flags & FIO_FILE_OPEN);
+
 	if (td->zone_bytes >= td->o.zone_size) {
 		td->zone_bytes = 0;
 		f->last_pos += td->o.zone_skip;