qcacld-3.0: Possible NULL ptr deref in dfs_phyerr_offload_event_handler

While handling WMI_DFS_RADAR_EVENTID event, driver invokes the
corresponding WMI handler dfs_phyerr_offload_event_handler. This
handler copies channel info from WMA context to local buffer chan.
As firmware sends the value of channel info to WMA context, it
could be NULL. Currently, there is no any NULL pointer validation
for chan present in dfs_phyerr_offload_event_handler before accessing
it. This results in a NULL pointer de-reference issue.

Add a sanity check for a NULL pointer before accessing chan in
dfs_phyerr_offload_event_handler.

Change-Id: I0789a25921df079222bea5203b583a20b404208a
CRs-Fixed: 2345088