handshake: clear expiration of pmksa in _steal_pmksa()

When pulling the pmksa out of the handshake object (to cache) we
also need to clear the expiration within the handshake itself. This
will prevent future attempts of setting the PMKSA into the handshake
due to the logic in handshake_state_set_pmksa():

/* checks for both expiration || pmksa being set */
if (s->expiration)
    return false;

src/handshake.c

@@ -1268,6 +1268,7 @@ static struct pmksa *handshake_state_steal_pmksa(struct handshake_state *s)
 
 	pmksa = l_new(struct pmksa, 1);
 	pmksa->expiration = s->expiration;
+	s->expiration = 0;
 	memcpy(pmksa->spa, s->spa, sizeof(s->spa));
 	memcpy(pmksa->aa, s->aa, sizeof(s->aa));
 	memcpy(pmksa->ssid, s->ssid, s->ssid_len);