dpdk: fix thread slot assignment

Commit 5592ec0 introduced a bug where ptv->slot was
assigned with the current one instead of the next one
in the list. This caused a Suricata crash in the DPDK mode.

Ticket: 6877

src/source-dpdk.c

@@ -568,7 +568,7 @@ static TmEcode ReceiveDPDKLoop(ThreadVars *tv, void *data, void *slot)
 {
     SCEnter();
     DPDKThreadVars *ptv = (DPDKThreadVars *)data;
-    ptv->slot = (TmSlot *)slot;
+    ptv->slot = ((TmSlot *)slot)->slot_next;
     TmEcode ret = ReceiveDPDKLoopInit(tv, ptv);
     if (ret != TM_ECODE_OK) {
         SCReturnInt(ret);