SSL: compatibility with BoringSSL.

BoringSSL changed SSL_set_tlsext_host_name() to be a real function
with a (const char *) argument, so it now triggers a warning due to
conversion from (u_char *).  Added an explicit cast to silence the
warning.

Prodded by Piotr Sikora, Alessandro Ghedini.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 4d5bea3..55a62ef 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1696,7 +1696,10 @@
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                    "upstream SSL server name: \"%s\"", name.data);
 
-    if (SSL_set_tlsext_host_name(c->ssl->connection, name.data) == 0) {
+    if (SSL_set_tlsext_host_name(c->ssl->connection,
+                                 (char *) name.data)
+        == 0)
+    {
         ngx_ssl_error(NGX_LOG_ERR, r->connection->log, 0,
                       "SSL_set_tlsext_host_name(\"%s\") failed", name.data);
         return NGX_ERROR;

src/stream/ngx_stream_proxy_module.c

diff --git a/src/stream/ngx_stream_proxy_module.c b/src/stream/ngx_stream_proxy_module.c
index 4231f97..7772eb6 100644
--- a/src/stream/ngx_stream_proxy_module.c
+++ b/src/stream/ngx_stream_proxy_module.c
@@ -1183,7 +1183,8 @@
     ngx_log_debug1(NGX_LOG_DEBUG_STREAM, s->connection->log, 0,
                    "upstream SSL server name: \"%s\"", name.data);
 
-    if (SSL_set_tlsext_host_name(u->peer.connection->ssl->connection, name.data)
+    if (SSL_set_tlsext_host_name(u->peer.connection->ssl->connection,
+                                 (char *) name.data)
         == 0)
     {
         ngx_ssl_error(NGX_LOG_ERR, s->connection->log, 0,