wifi: ath10k: fix NULL pointer dereference in ath10k_wmi_tlv_op_pull_mgmt_tx_compl_ev()

We should check whether the WMI_TLV_TAG_STRUCT_MGMT_TX_COMPL_EVENT tlv is
present before accessing it, otherwise a null pointer deference error will
occur.

Fixes: dc405152

 ("ath10k: handle mgmt tx completion event")
Signed-off-by: Xingyuan Mo <hdthky0@gmail.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20231208043433.271449-1-hdthky0@gmail.com