QSslDiffieHellmanParameters: fix mem-leak

Says ASAN:

    Direct leak of 524 byte(s) in 1 object(s) allocated from:
    #0 0x7f708f0a67cf in __interceptor_malloc ../../../../gcc/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x7f707d94bf9e in CRYPTO_malloc crypto/mem.c:196
    #2 0x7f707d7bd248 in asn1_item_flags_i2d crypto/asn1/tasn_enc.c:65
    #3 0x7f707d7bd1b7 in ASN1_item_i2d crypto/asn1/tasn_enc.c:45
    #4 0x7f707d85b7be in i2d_DHparams crypto/dh/dh_asn1.c:54
    #5 0x7f7075a82223 in q_i2d_DHparams(dh_st*, unsigned char**) qsslsocket_openssl_symbols.cpp:435
    #6 0x7f7075a82223 in QTlsBackendOpenSSL::dhParametersFromPem(QByteArray const&, QByteArray*) const qssldiffiehellmanparameters_openssl.cpp:139
    #7 0x7f708ca9b588 in QSslDiffieHellmanParametersPrivate::initFromPem(QByteArray const&) qssldiffiehellmanparameters.cpp:285
    #8 0x7f708ca9b588 in QSslDiffieHellmanParameters::fromEncoded(QByteArray const&, QSsl::EncodingFormat) qssldiffiehellmanparameters.cpp:94
    #9 0x55fd8a545ebe in tst_QSslDiffieHellmanParameters::constructionPEM() tst_qssldiffiehellmanparameters.cpp:98
    [...]

The pointer returned in the out-parameter of a i2d_DHparams() call is
supposed to be OPENSSL_free()ed by the user (this is not at all
obvious from the docs¹, but an SO answer² indicates that's how it
should be (as well as asan stopping from complaining with this
patch applied)).

¹ https://www.openssl.org/docs/man3.1/man3/i2d_DHparams.html
² https://stackoverflow.com/a/53563669.

Amends 2cf63c71ebe139890526057dcc51b24ea6df6c30.

[ChangeLog][QtNetwork][SSL] Fixed a memory leak in parsing of
PEM-encoded Diffie-Hellman parameters.

Change-Id: I9ed4a26c4676db1c0d54a1945a4fb5014ce568cd
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
(cherry picked from commit 676087ef1f7cc885d51256ec30e242d972dccb65)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/tls/openssl/qssldiffiehellmanparameters_openssl.cpp

@@ -137,6 +137,7 @@ int QTlsBackendOpenSSL::dhParametersFromPem(const QByteArray &pem, QByteArray *d
         if (isSafeDH(dh)) {
             char *buf = nullptr;
             const int len = q_i2d_DHparams(dh, reinterpret_cast<unsigned char **>(&buf));
+            const auto freeBuf = qScopeGuard([&] { q_OPENSSL_free(buf); });
             if (len > 0)
                 *data = QByteArray(buf, len);
             else