macsec_linux: Fix NULL pointer dereference on error cases

In case wpa_supplicant is using driver_macsec_linux, but macsec module
is not (yet) loaded in the kernel, nl_socket_alloc() fails and drv->sk
is NULL. In this case, don't call libnl functions rntl_link_add() or
rtnl_link_change() using such NULL pointer, to prevent program from
getting segmentation faults like:

 Program received signal SIGSEGV, Segmentation fault.
 nl_socket_get_local_port (sk=sk@entry=0x0) at socket.c:365
 365             if (sk->s_local.nl_pid == 0) {
 (gdb) p sk
 $1 = (const struct nl_sock *) 0x0
 (gdb) bt
 #0  nl_socket_get_local_port (sk=sk@entry=0x0) at socket.c:365
 #1  0x00007ffff79c56a0 in nl_complete_msg (sk=sk@entry=0x0,
  msg=msg@entry=0x55555595a1f0) at nl.c:491
 #2  0x00007ffff79c56d1 in nl_send_auto (sk=sk@entry=0x0,
  msg=msg@entry=0x55555595a1f0) at nl.c:522
 #3  0x00007ffff79c652f in nl_send_sync (sk=sk@entry=0x0,
  msg=0x55555595a1f0) at nl.c:556
 #4  0x00007ffff755faf5 in rtnl_link_add (sk=0x0,
  link=link@entry=0x55555595b0f0, flags=flags@entry=1024) at route/link.c:1548
 #5  0x000055555567a298 in macsec_drv_create_transmit_sc (priv=0x55555593b130,
  sc=0x55555593b320, conf_offset=<optimized out>) at ../src/drivers/driver_macsec_linux.c:998

Signed-off-by: Davide Caratti <davide.caratti@gmail.com>

src/drivers/driver_macsec_linux.c

@@ -168,6 +168,9 @@ static int try_commit(struct macsec_drv_data *drv)
 {
 	int err;
 
+	if (!drv->sk)
+		return 0;
+
 	if (!drv->link)
 		return 0;
 
@@ -982,6 +985,11 @@ static int macsec_drv_create_transmit_sc(
 
 	wpa_printf(MSG_DEBUG, "%s", __func__);
 
+	if (!drv->sk) {
+		wpa_printf(MSG_ERROR, DRV_PREFIX "NULL rtnl socket");
+		return -1;
+	}
+
 	link = rtnl_link_macsec_alloc();
 	if (!link) {
 		wpa_printf(MSG_ERROR, DRV_PREFIX "couldn't allocate link");
@@ -1048,6 +1056,9 @@ static int macsec_drv_delete_transmit_sc(void *priv, struct transmit_sc *sc)
 
 	wpa_printf(MSG_DEBUG, "%s", __func__);
 
+	if (!drv->sk)
+		return 0;
+
 	if (!drv->created_link) {
 		rtnl_link_put(drv->link);
 		drv->link = NULL;