MINOR: quic-be: Avoid SSL context unreachable code without USE_QUIC_OPENSSL_COMPAT

MINOR: quic-be: Avoid SSL context unreachable code without USE_QUIC_OPENSSL_COMPAT

This commit added a "err" C label reachable only with USE_QUIC_OPENSSL_COMPAT:

   MINOR: quic-be: Missing callbacks initializations (USE_QUIC_OPENSSL_COMPAT)

leading coverity to warn this:

*** CID 1611481:         Control flow issues  (UNREACHABLE)
/src/quic_ssl.c: 802             in ssl_quic_srv_new_ssl_ctx()
796     		goto err;
797     #endif
798
799      leave:
800     	TRACE_LEAVE(QUIC_EV_CONN_NEW);
801     	return ctx;
>>>     CID 1611481:         Control flow issues  (UNREACHABLE)
>>>     This code cannot be reached: "err:
SSL_CTX_free(ctx);".
802      err:
803     	SSL_CTX_free(ctx);
804     	ctx = NULL;
805     	TRACE_DEVEL("leaving on error", QUIC_EV_CONN_NEW);
806     	goto leave;
807     }

The less intrusive (without #ifdef) way to fix this it to add a "goto err"
statement from the code part which is reachable without USE_QUIC_OPENSSL_COMPAT.

Thank you to @chipitsine for having reported this issue in GH #3003.