UPSTREAM: wifi: cfg80211: ensure length byte is present before access

commit 567e14e39e8f8c6997a1378bc3be615afca86063 upstream.

When iterating the elements here, ensure the length byte is
present before checking it to see if the entire element will
fit into the buffer.

Longer term, we should rewrite this code using the type-safe
element iteration macros that check all of this.

Bug: 254180332
Fixes: 0b8fb823 ("cfg80211: Parsing of Multiple BSSID information in scanning")
Reported-by: Soenke Huster <shuster@seemoo.tu-darmstadt.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I2ff6215eabd24d0fbb57a9467cdd548523ffbd9b