qcacld-3.0: Fix possible NULL pointer de-reference

Fix possible NULL pointer de-reference in functions
peer_unmap_timer_handler().

Change-Id: I2ec6af12b1f8232a247729fd658c2d51ff163405
CRs-Fixed: 2512610

core/dp/txrx/ol_txrx.c

@@ -3181,6 +3181,9 @@ void peer_unmap_timer_handler(void *data)
 {
 	ol_txrx_peer_handle peer = (ol_txrx_peer_handle)data;
 
+	if (!peer)
+		return;
+
 	ol_txrx_err("all unmap events not received for peer %pK, ref_cnt %d",
 		    peer, qdf_atomic_read(&peer->ref_cnt));
 	ol_txrx_err("peer %pK ("QDF_MAC_ADDR_STR")",