qcacld-3.0: Check adapter validity for IPA pending events

For stop ap case at last client disconnect Host sends HTT message
to FW for IPA RX pipe suspend. In case of delay in corresponding
HTT response from FW, ap disconnect IPA WLAN event will be added
to pending queue and the event is processed after receiving the FW
HTT response for RX pipe suspend. If the FW response arrives after
close adapter of SAP interface processing IPA pending events will
lead to invalid memory access as the adapter reference stored as
part of pending event information will be deallocated as part of
close adapter.

In this change check if the adapter reference held by the pending
event is valid.

Change-Id: I9b0e4b8f58bb50062e6f2ef360ac2c53055ad9f7
CRs-Fixed: 2290027