Smatch spot a possible NULL pointer dereference. Fix it.

__iwl_mvm_mac_set_key can be called with sta = NULL
Also add a NULL pointer check after memory allocation.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Link: https://lore.kernel.org/r/20211130105951.85539-1-emmanuel.grumbach@intel.comSigned-off-by: Luca Coelho <luciano.coelho@intel.com>