detect/alert: directly increment alerts.discarded

In the unlikely case of AlertQueueExpand failure, we were incrementing
the discarded alerts stats in AlertQueueAppend via the Packet member in the
DetectEngineThreadCtx, which may not be initialized yet.

Bug #5353

src/detect-engine-alert.c

@@ -268,7 +268,7 @@ void AlertQueueAppend(DetectEngineThreadCtx *det_ctx, const Signature *s, Packet
         /* we must grow the alert queue */
         if (pos == AlertQueueExpand(det_ctx)) {
             /* this means we failed to expand the queue */
-            det_ctx->p->alerts.discarded++;
+            p->alerts.discarded++;
             return;
         }
     }