use correct type cast

src/core/ngx_string.c

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 33857fe..11f1cf2 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -182,7 +182,7 @@
                     continue;
 
                 case '*':
-                    slen = va_arg(args, u_int);
+                    slen = va_arg(args, size_t);
                     fmt++;
                     continue;