qcacmn: Check pdevhandle before processing htt msg

Add null check for pdevhandle before processing htt msg.
This is added for polled mode to avoid race between pdev detach
and CE poll timer stop.

Change-Id: I925e70985bd11e05e0679ad0107493aaefe8a273
CRs-fixed: 2352564

dp/wifi3.0/dp_htt.c

@@ -2578,6 +2578,9 @@ static bool dp_txrx_ppdu_stats_handler(struct dp_soc *soc,
 	struct ppdu_info *ppdu_info = NULL;
 	bool free_buf = true;
 
+	if (!pdev)
+		return true;
+
 	if (!pdev->enhanced_stats_en && !pdev->tx_sniffer_enable &&
 			!pdev->mcopy_mode)
 		return free_buf;