ath10k: fix peer stats null pointer dereference

There was a race condition in SMP that an ath10k_peer was created but its
member sta was null. Following are procedures of ath10k_peer creation and
member sta access in peer statistics path.

    1. Peer creation:
        ath10k_peer_create()
            =>ath10k_wmi_peer_create()
                =>ath10k_wait_for_peer_created()
                ...

        # another kernel path, RX from firmware
        ath10k_htt_t2h_msg_handler()
        =>ath10k_peer_map_event()
                =>wake_up()
                # ar->peer_map[id] = peer //add peer to map

        #wake up original path from waiting
                ...
                # peer->sta = sta //sta assignment

    2.  RX path of statistics
        ath10k_htt_t2h_msg_handler()
            =>ath10k_update_per_peer_tx_stats()
                =>ath10k_htt_fetch_peer_stats()
                # peer->sta //sta accessing

Any access of peer->sta after peer was added to peer_map but before sta was
assigned could cause a null pointer issue. And because these two steps are
asynchronous, no proper lock can protect them. So both peer and sta need to
be checked before access.

Tested: QCA9984 with firmware ver 10.4-3.9.0.1-00005
Signed-off-by: Zhi Chen <zhichen@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>