Add support for replaying blktrace trim/discard

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/init.c b/init.c
index ff7da40..90e2063 100644
--- a/init.c
+++ b/init.c
@@ -194,6 +194,8 @@
 	unsigned int bs = td->o.min_bs[ddir];
 	unsigned long long bytes_per_sec;
 
+	assert(ddir_rw(ddir));
+
 	if (td->o.rate[ddir])
 		bytes_per_sec = td->o.rate[ddir];
 	else