defrag: use util function for timeout

To fix timestamp overflow as found by oss-fuzz
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=44608

fixu

src/defrag.c

@@ -628,8 +628,7 @@ DefragInsertFrag(ThreadVars *tv, DecodeThreadVars *dtv, DefragTracker *tracker,
     }
 
     /* Update timeout. */
-    tracker->timeout.tv_sec = p->ts.tv_sec + tracker->host_timeout;
-    tracker->timeout.tv_usec = p->ts.tv_usec;
+    tracker->timeout = TimevalWithSeconds(&p->ts, tracker->host_timeout);
 
     Frag *prev = NULL, *next = NULL;
     bool overlap = false;