t/btrace2fio: fix divide-by-zero in rate calculation

If usec is less than 1000, it'd crash.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/t/btrace2fio.c b/t/btrace2fio.c
index bf67ed0..efe25c4 100644
--- a/t/btrace2fio.c
+++ b/t/btrace2fio.c
@@ -549,8 +549,12 @@
 	if (!usec)
 		return 0;
 
+	usec /= 1000;
+	if (!usec)
+		return 0;
+
 	val = o->kb[rw] * 1000ULL;
-	return val / (usec / 1000ULL);
+	return val / usec;
 }
 
 static uint64_t o_first_ttime(struct btrace_out *o)