fix incorrect interval check in sync from disk stage (AFLplusplus#2408)

* fix incorrect interval check in sync from disk stage

* simplify check

libafl/src/stages/sync.rs

@@ -97,7 +97,7 @@ where
             .map(|m| m.last_time);
 
         if let Some(last) = last {
-            if (last + self.interval) < current_time() {
+            if current_time().saturating_sub(last) < self.interval {
                 return Ok(());
             }
         }