BUG/MINOR: ssl: fix potential memory leak in ssl_sock_load_dh_params()

Roberto Guimaraes reported that Valgrind complains about a leak
in ssl_get_dh_1024().
This is caused caused by an oversight in ssl_sock_load_dh_params(),
where local_dh_1024 is always replaced by a new DH object even if
it already holds one. This patch simply checks whether local_dh_1024
is NULL before calling ssl_get_dh_1024().
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index f247618..e5a6f0a 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -1638,7 +1638,9 @@
 
 		if (global.tune.ssl_default_dh_param <= 1024) {
 			/* we are limited to DH parameter of 1024 bits anyway */
-			local_dh_1024 = ssl_get_dh_1024();
+			if (local_dh_1024 == NULL)
+				local_dh_1024 = ssl_get_dh_1024();
+
 			if (local_dh_1024 == NULL)
 				goto end;