Check for NULL when freeing the QUIC_TLS object

Free functions are expected to be tolerant of a NULL pointer being passed.

Fixes the problem in
https://github.com/openssl/openssl/pull/21668#issuecomment-1782718328

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22536)

ssl/quic/quic_tls.c

@@ -654,6 +654,8 @@ QUIC_TLS *ossl_quic_tls_new(const QUIC_TLS_ARGS *args)
 
 void ossl_quic_tls_free(QUIC_TLS *qtls)
 {
+    if (qtls == NULL)
+        return;
     OSSL_ERR_STATE_free(qtls->error_state);
     OPENSSL_free(qtls);
 }