Moved a request assert to an earlier location

An assert protecting from a non-null request was happening too late in pkt_COOKIE_handler from
mpid_nem_lmt.c. This patch moves it to an earlier location so that it's checked before it's first
used.

Reported by Dmitry Polyakov.

src/mpid/ch3/channels/nemesis/src/mpid_nem_lmt.c

@@ -462,13 +462,14 @@ static int pkt_COOKIE_handler(MPIDI_VC_t *vc, MPIDI_CH3_Pkt_t *pkt, MPIDI_msg_sz
 
     if (cookie_pkt->from_sender) {
         MPID_Request_get_ptr(cookie_pkt->receiver_req_id, req);
+        MPIU_Assert(req != NULL);
         req->ch.lmt_req_id = cookie_pkt->sender_req_id;
     }
     else {
         MPID_Request_get_ptr(cookie_pkt->sender_req_id, req);
+        MPIU_Assert(req != NULL);
         req->ch.lmt_req_id = cookie_pkt->receiver_req_id;
     }
-    MPIU_Assert(req != NULL);
 
     if (cookie_pkt->cookie_len != 0)
     {