mac80211: add length check in ieee80211_is_robust_mgmt_frame()

A few places weren't checking that the frame passed to the
function actually has enough data even though the function
clearly documents it must have a payload byte. Make this
safer by changing the function to take an skb and checking
the length inside. The old version is preserved for now as
the rtl* drivers use it and don't have a correct skb.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/drivers/net/wireless/rtlwifi/rtl8192ce/trx.c b/drivers/net/wireless/rtlwifi/rtl8192ce/trx.c
index 52abf0a..114858d 100644
--- a/drivers/net/wireless/rtlwifi/rtl8192ce/trx.c
+++ b/drivers/net/wireless/rtlwifi/rtl8192ce/trx.c
@@ -393,7 +393,7 @@
 			/* In testing, hdr was NULL here */
 			return false;
 		}
-		if ((ieee80211_is_robust_mgmt_frame(hdr)) &&
+		if ((_ieee80211_is_robust_mgmt_frame(hdr)) &&
 		    (ieee80211_has_protected(hdr->frame_control)))
 			rx_status->flag &= ~RX_FLAG_DECRYPTED;
 		else