fix building by msvc introduced in r2223

src/core/ngx_conf_file.c

diff --git a/src/core/ngx_conf_file.c b/src/core/ngx_conf_file.c
index c7e155c..d968316 100644
--- a/src/core/ngx_conf_file.c
+++ b/src/core/ngx_conf_file.c
@@ -508,7 +508,7 @@
                 ngx_memcpy(b->start, start, len);
             }
 
-            size = file_size - cf->conf_file->file.offset;
+            size = (ssize_t) (file_size - cf->conf_file->file.offset);
 
             if (size > b->end - (b->start + len)) {
                 size = b->end - (b->start + len);