eap-mschapv2: Fix uninitialized use of memory

When the response structure is generated, not all of the memory was
initialized to 0.

==1045== Syscall param socketcall.sendto(msg) points to uninitialised byte(s)
==1045==    at 0x5134D52: send (in /lib64/libc-2.25.so)
==1045==    by 0x168AB5: l_checksum_update (checksum.c:338)
==1045==    by 0x186777: tls_write_mac (tls-record.c:58)
==1045==    by 0x1869D1: tls_tx_record_plaintext (tls-record.c:120)
==1045==    by 0x186DEA: tls_tx_record (tls-record.c:201)
==1045==    by 0x185A3B: l_tls_write (tls.c:2064)
==1045==    by 0x14584F: eap_ttls_eap_tx_packet (eap-ttls.c:321)
==1045==    by 0x14236C: eap_send_response (eap.c:165)
==1045==    by 0x147904: eap_mschapv2_send_response (eap-mschapv2.c:468)
==1045==    by 0x147A10: eap_mschapv2_handle_challenge (eap-mschapv2.c:492)
==1045==    by 0x147E9A: eap_mschapv2_handle_request (eap-mschapv2.c:615)
==1045==    by 0x142693: __eap_handle_request (eap.c:240)
==1045==  Address 0x1ffeffe7f9 is on thread 1's stack
==1045==  in frame #4, created by tls_tx_record (tls-record.c:177)
==1045==  Uninitialised value was created by a stack allocation
==1045==    at 0x1477AE: eap_mschapv2_send_response (eap-mschapv2.c:443)
==1045==
==1045== Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s)
==1045==    at 0x5134E3B: sendmsg (in /lib64/libc-2.25.so)
==1045==    by 0x17F691: operate_cipher (cipher.c:356)
==1045==    by 0x17F9D8: l_cipher_encrypt (cipher.c:446)
==1045==    by 0x186BAA: tls_tx_record_plaintext (tls-record.c:152)
==1045==    by 0x186DEA: tls_tx_record (tls-record.c:201)
==1045==    by 0x185A3B: l_tls_write (tls.c:2064)
==1045==    by 0x14584F: eap_ttls_eap_tx_packet (eap-ttls.c:321)
==1045==    by 0x14236C: eap_send_response (eap.c:165)
==1045==    by 0x147904: eap_mschapv2_send_response (eap-mschapv2.c:468)
==1045==    by 0x147A10: eap_mschapv2_handle_challenge (eap-mschapv2.c:492)
==1045==    by 0x147E9A: eap_mschapv2_handle_request (eap-mschapv2.c:615)
==1045==    by 0x142693: __eap_handle_request (eap.c:240)
==1045==  Address 0x1ffeffe7f9 is on thread 1's stack
==1045==  in frame #4, created by tls_tx_record (tls-record.c:177)
==1045==  Uninitialised value was created by a stack allocation
==1045==    at 0x1477AE: eap_mschapv2_send_response (eap-mschapv2.c:443)
==1045==
1 file changed