HTTP/2: unbreak build on MSVC.

src/http/v2/ngx_http_v2.c

diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
index cd0243c..208f0f3 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -3544,7 +3544,7 @@
     }
 
     if (r->request_body_no_buffering) {
-        size = len - h2scf->preread_size;
+        size = (size_t) len - h2scf->preread_size;
 
     } else {
         stream->no_flow_control = 1;