BUG/MINOR: ssl: don't initialize the keylog callback when not required

The registering of the keylog callback seems to provoke a loss of
performance. Disable the registration as well as the fetches if
tune.ssl.keylog is off.

Must be backported as far as 2.2.
diff --git a/src/ssl_sample.c b/src/ssl_sample.c
index 7eee065..30a6162 100644
--- a/src/ssl_sample.c
+++ b/src/ssl_sample.c
@@ -1855,6 +1855,9 @@
 	char *src = NULL;
 	const char *sfx;
 
+	if (global_ssl.keylog <= 0)
+		return 0;
+
 	conn = (kw[4] != 'b') ? objt_conn(smp->sess->origin) :
 	       smp->strm ? sc_conn(smp->strm->scb) : NULL;
 
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index e066f28..2a4d644 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -4979,7 +4979,9 @@
 	SSL_CTX_set_msg_callback(ctx, ssl_sock_msgcbk);
 #endif
 #ifdef HAVE_SSL_KEYLOG
-	SSL_CTX_set_keylog_callback(ctx, SSL_CTX_keylog);
+	/* only activate the keylog callback if it was required to prevent performance loss */
+	if (global_ssl.keylog > 0)
+		SSL_CTX_set_keylog_callback(ctx, SSL_CTX_keylog);
 #endif
 
 #if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)