wifi: iwlwifi: mvm: Add NULL check before dereferencing the pointer

While vif pointers are protected by the corresponding "*active"
fields, static checkers can get confused sometimes. Add an explicit
check.

Signed-off-by: Mukesh Sisodiya <mukesh.sisodiya@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230614154951.78749ae91fb5.Id3c05d13eeee6638f0930f750e93fb928d5c9dee@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>