Stream: fixed MSVC compilation warning.

Thanks to itpp2012.

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 ebbe027..761e5f7 100644
--- a/src/stream/ngx_stream_proxy_module.c
+++ b/src/stream/ngx_stream_proxy_module.c
@@ -1063,7 +1063,7 @@
                 }
 
                 if (size > (size_t) limit) {
-                    size = limit;
+                    size = (size_t) limit;
                 }
             }