eap-wsc: fix valgrind warning

Valgrind does not like uninitialized bytes used in a syscall. In this
case the buffer is an out buffer but since valgrind doesn't know that
it complains. Initializing to zero fixes the warning:

Syscall param socketcall.sendto(msg) points to uninitialised byte(s)
    at 0x5162C4D: send (send.c:28)
    by 0x457AF4: l_checksum_update (checksum.c:319)
    by 0x43C03C: eap_wsc_handle_m2 (eap-wsc.c:842)
    by 0x43CD33: eap_wsc_handle_request (eap-wsc.c:1048)
    by 0x43A3A7: __eap_handle_request.part.0 (eap.c:266)
    by 0x41A426: eapol_rx_packet.part.12 (eapol.c:2262)
    by 0x41B536: __eapol_rx_packet (eapol.c:2650)
    by 0x407C80: netdev_control_port_frame_event (netdev.c:3542)
    by 0x407C80: netdev_unicast_notify (netdev.c:3684)
    by 0x4598C5: dispatch_unicast_watches (genl.c:899)
    by 0x4598C5: process_unicast (genl.c:918)
    by 0x4598C5: received_data (genl.c:1039)
    by 0x456452: io_callback (io.c:126)
    by 0x45569D: l_main_iterate (main.c:473)
    by 0x45576B: l_main_run (main.c:520)
  Address 0x1ffeffe290 is on thread 1's stack
in frame #2, created by eap_wsc_handle_m2 (eap-wsc.c:797)

src/eap-wsc.c

@@ -797,7 +797,7 @@ static void eap_wsc_handle_m2(struct eap_state *eap,
 {
 	struct eap_wsc_state *wsc = eap_get_data(eap);
 	struct l_key *remote_public;
-	uint8_t shared_secret[192];
+	uint8_t shared_secret[192] = { 0 };
 	size_t shared_secret_len = sizeof(shared_secret);
 	struct l_checksum *sha256;
 	uint8_t dhkey[32];