Clear beacon ie pointer and ie length after free
in order to prevent double free.

==================================================================
BUG: KASAN: double-free or invalid-free \
in ieee80211_ibss_leave+0x83/0xe0 net/mac80211/ibss.c:1876

CPU: 0 PID: 8472 Comm: syz-executor100 Not tainted 5.11.0-rc6-syzkaller #0
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0x107/0x163 lib/dump_stack.c:120
 print_address_description.constprop.0.cold+0x5b/0x2c6 mm/kasan/report.c:230
 kasan_report_invalid_free+0x51/0x80 mm/kasan/report.c:355
 ____kasan_slab_free+0xcc/0xe0 mm/kasan/common.c:341
 kasan_slab_free include/linux/kasan.h:192 [inline]
 __cache_free mm/slab.c:3424 [inline]
 kfree+0xed/0x270 mm/slab.c:3760
 ieee80211_ibss_leave+0x83/0xe0 net/mac80211/ibss.c:1876
 rdev_leave_ibss net/wireless/rdev-ops.h:545 [inline]
 __cfg80211_leave_ibss+0x19a/0x4c0 net/wireless/ibss.c:212
 __cfg80211_leave+0x327/0x430 net/wireless/core.c:1172
 cfg80211_leave net/wireless/core.c:1221 [inline]
 cfg80211_netdev_notifier_call+0x9e8/0x12c0 net/wireless/core.c:1335
 notifier_call_chain+0xb5/0x200 kernel/notifier.c:83
 call_netdevice_notifiers_info+0xb5/0x130 net/core/dev.c:2040
 call_netdevice_notifiers_extack net/core/dev.c:2052 [inline]
 call_netdevice_notifiers net/core/dev.c:2066 [inline]
 __dev_close_many+0xee/0x2e0 net/core/dev.c:1586
 __dev_close net/core/dev.c:1624 [inline]
 __dev_change_flags+0x2cb/0x730 net/core/dev.c:8476
 dev_change_flags+0x8a/0x160 net/core/dev.c:8549
 dev_ifsioc+0x210/0xa70 net/core/dev_ioctl.c:265
 dev_ioctl+0x1b1/0xc40 net/core/dev_ioctl.c:511
 sock_do_ioctl+0x148/0x2d0 net/socket.c:1060
 sock_ioctl+0x477/0x6a0 net/socket.c:1177
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl fs/ioctl.c:739 [inline]
 __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:739
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported-by: syzbot+93976391bf299d425f44@syzkaller.appspotmail.com
Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Link: https://lore.kernel.org/r/20210213133653.367130-1-markus.theil@tu-ilmenau.deSigned-off-by: Johannes Berg <johannes.berg@intel.com>