SSL: explicitly set maximum version (ticket #1654).

With maximum version explicitly set, TLSv1.3 will not be unexpectedly
enabled if nginx compiled with OpenSSL 1.1.0 (without TLSv1.3 support)
will be run with OpenSSL 1.1.1 (with TLSv1.3 support).

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 7dcd1cc..c4b51b5 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -330,6 +330,11 @@
     }
 #endif
 
+#ifdef SSL_CTX_set_min_proto_version
+    SSL_CTX_set_min_proto_version(ssl->ctx, 0);
+    SSL_CTX_set_max_proto_version(ssl->ctx, TLS1_2_VERSION);
+#endif
+
 #ifdef TLS1_3_VERSION
     SSL_CTX_set_min_proto_version(ssl->ctx, 0);
     SSL_CTX_set_max_proto_version(ssl->ctx, TLS1_3_VERSION);