defrag: be sure to output NULL tracker

Coverity 720337 pointed out a use after free. We can't be dependent
to HashListTableAdd outputting a NULL tracker.

src/defrag.c

@@ -1169,6 +1169,7 @@ DefragGetTracker(ThreadVars *tv, DecodeThreadVars *dtv, DefragContext *dc,
             SCMutexLock(&dc->tracker_pool_lock);
             PoolReturn(dc->tracker_pool, tracker);
             SCMutexUnlock(&dc->tracker_pool_lock);
+            tracker = NULL;
             goto done;
         }
     }