Fix interval reporting for zero runtime

-r 0 makes us run forever, we shouldn't set the interval to zero in this
case.

Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/simoop.c b/simoop.c
index 81204b9..7c7fc04 100644
--- a/simoop.c
+++ b/simoop.c
@@ -1929,7 +1929,7 @@
 	save_vmstat_rates(&vmstat_info);
 	save_instant_vmstat_rates(&vmstat_info);
 
-	if (interval_seconds > runtime)
+	if (interval_seconds > runtime && runtime > 0)
 		interval_seconds = runtime;
 
 	while(!stopping) {