wifi: avoid offset calculation on NULL pointer

ieee80211_he_6ghz_oper() can be passed a NULL pointer
and checks for that, but already did the calculation
to inside of it before. Move it after the check.
Signed-off-by: Michael-CY Lee <michael-cy.lee@mediatek.com>
Link: https://lore.kernel.org/r/20231122030237.31276-1-michael-cy.lee@mediatek.com
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/linux/ieee80211.h

@@ -2830,12 +2830,14 @@ ieee80211_he_oper_size(const u8 *he_oper_ie)
 static inline const struct ieee80211_he_6ghz_oper *
 ieee80211_he_6ghz_oper(const struct ieee80211_he_operation *he_oper)
 {
-	const u8 *ret = (const void *)&he_oper->optional;
+	const u8 *ret;
 	u32 he_oper_params;
 
 	if (!he_oper)
 		return NULL;
 
+	ret = (const void *)&he_oper->optional;
+
 	he_oper_params = le32_to_cpu(he_oper->he_oper_params);
 
 	if (!(he_oper_params & IEEE80211_HE_OPERATION_6GHZ_OP_INFO))