Fix DHCP/NDISC snoop deinit followed by failing re-init

It was possible to hit a double-free on the l2_packet socket if
initialization of DHCP/NDISC snoop failed on a hostapd interface that
had previously had those enabled successfully. Fix this by clearing the
l2_packet pointers during deinit.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/ap/dhcp_snoop.c

@@ -154,4 +154,5 @@ int dhcp_snoop_init(struct hostapd_data *hapd)
 void dhcp_snoop_deinit(struct hostapd_data *hapd)
 {
 	l2_packet_deinit(hapd->sock_dhcp);
+	hapd->sock_dhcp = NULL;
 }

src/ap/ndisc_snoop.c

@@ -182,4 +182,5 @@ int ndisc_snoop_init(struct hostapd_data *hapd)
 void ndisc_snoop_deinit(struct hostapd_data *hapd)
 {
 	l2_packet_deinit(hapd->sock_ndisc);
+	hapd->sock_ndisc = NULL;
 }