hostapd: P2P: Fix a corner case in peer addition based on PD Request

hostapd: P2P: Fix a corner case in peer addition based on PD Request

p2p_add_device() may remove the oldest entry if there is no room in the
peer table for a new peer. This would result in any pointer to that
removed entry becoming stale. A corner case with an invalid PD Request
frame could result in such a case ending up using (read+write) freed
memory. This could only by triggered when the peer table has reached its
maximum size and the PD Request frame is received from the P2P Device
Address of the oldest remaining entry and the frame has incorrect P2P
Device Address in the payload.

Fix this by fetching the dev pointer again after having called
p2p_add_device() so that the stale pointer cannot be used.

This fixes the following security vulnerabilities/bugs:

- CVE-2021-27803 - A vulnerability was discovered in how p2p/p2p_pd.c
  in wpa_supplicant before 2.10 processes P2P (Wi-Fi Direct) provision
  discovery requests. It could result in denial of service or other
  impact (potentially execution of arbitrary code), for an attacker
  within radio range.

Fixes: 17bef1e97a50 ("P2P: Add peer entry based on Provision Discovery Request")
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>