wifi: ath10k: Check return value of ath10k_get_arvif() in ath10k_wmi_event_tdls_peer()

Return value of a function ath10k_get_arvif() is dereferenced without
checking for null in ath10k_wmi_event_tdls_peer(), but it is usually checked
for this function.

Make ath10k_wmi_event_tdls_peer() do check retval of ath10k_get_arvif().

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Peter Kosyh <pkosyh@yandex.ru>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20221003091217.322598-1-pkosyh@yandex.ru