Request body: do not create temp file if there's nothing to write.

Do this only when the entire request body is empty and
r->request_body_in_file_only is set.

The issue manifested itself with missing warning "a client request body is
buffered to a temporary file" when the entire rb->buf is full and all buffers
are delayed by a filter.

src/http/ngx_http_request_body.c

@@ -1309,7 +1309,7 @@ ngx_http_request_body_save_filter(ngx_http_request_t *r, ngx_chain_t *in)
 
     if (rb->rest > 0) {
 
-        if (rb->buf && rb->buf->last == rb->buf->end
+        if (rb->bufs && rb->buf && rb->buf->last == rb->buf->end
             && ngx_http_write_request_body(r) != NGX_OK)
         {
             return NGX_HTTP_INTERNAL_SERVER_ERROR;