Fix a minor memory sanitizer issue

Here the undefined value "npa" passed to a function
WPACKET_sub_memcpy_u16(pkt, npa, npalen).
However the value is not really used, because "npalen" is zero,
but the call statememt itself is considered an invalid operation
by the new sanitizer.

The original sanitizer error report was:

==49175==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x55a276b29d6f in tls_construct_stoc_next_proto_neg /home/runner/work/openssl/openssl/ssl/statem/extensions_srvr.c:1518:21
    #1 0x55a276b15d7d in tls_construct_extensions /home/runner/work/openssl/openssl/ssl/statem/extensions.c:909:15
    #2 0x55a276b513dc in tls_construct_server_hello /home/runner/work/openssl/openssl/ssl/statem/statem_srvr.c:2471:10
    #3 0x55a276b2e160 in write_state_machine /home/runner/work/openssl/openssl/ssl/statem/statem.c:896:26
    #4 0x55a276b2e160 in state_machine /home/runner/work/openssl/openssl/ssl/statem/statem.c:490:21
    #5 0x55a276b2f562 in ossl_statem_accept /home/runner/work/openssl/openssl/ssl/statem/statem.c:309:12
    #6 0x55a276a9f867 in SSL_do_handshake /home/runner/work/openssl/openssl/ssl/ssl_lib.c:4890:19
    #7 0x55a276a9f605 in SSL_accept /home/runner/work/openssl/openssl/ssl/ssl_lib.c:2169:12
    #8 0x55a276a3d4db in create_bare_ssl_connection /home/runner/work/openssl/openssl/test/helpers/ssltestlib.c:1281:24
    #9 0x55a276a3d7cb in create_ssl_connection /home/runner/work/openssl/openssl/test/helpers/ssltestlib.c:1350:10
    #10 0x55a276a64c0b in test_npn /home/runner/work/openssl/openssl/test/sslapitest.c:12266:14
    #11 0x55a276b9fc20 in run_tests /home/runner/work/openssl/openssl/test/testutil/driver.c:377:21
    #12 0x55a276ba0b10 in main /home/runner/work/openssl/openssl/test/testutil/main.c:31:15

Reviewed-by: Saša Nedvědický <[email protected]>
Reviewed-by: Tomas Mraz <[email protected]>
(Merged from openssl#26269)

test/sslapitest.c

@@ -12194,6 +12194,7 @@ static int npn_advert_cb(SSL *ssl, const unsigned char **out,
         return SSL_TLSEXT_ERR_OK;
 
     case 1:
+        *out = NULL;
         *outlen = 0;
         return SSL_TLSEXT_ERR_OK;