mac80211: fix possible NULL pointerderef in obss pd code

he_spr_ie_elem is dereferenced before the NULL check. fix this by moving
the assignment after the check.

fixes commit 697f6c50 ("mac80211: propagate HE operation info into
bss_conf")

This was reported by the static code checker.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20190813070712.25509-1-john@phrozen.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/he.c

@@ -72,12 +72,13 @@ ieee80211_he_spr_ie_to_bss_conf(struct ieee80211_vif *vif,
 {
 	struct ieee80211_he_obss_pd *he_obss_pd =
 					&vif->bss_conf.he_obss_pd;
-	const u8 *data = he_spr_ie_elem->optional;
+	const u8 *data;
 
 	memset(he_obss_pd, 0, sizeof(*he_obss_pd));
 
 	if (!he_spr_ie_elem)
 		return;
+	data = he_spr_ie_elem->optional;
 
 	if (he_spr_ie_elem->he_sr_control &
 	    IEEE80211_HE_SPR_NON_SRG_OFFSET_PRESENT)