fix case when the first try is shorter then URI

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 6a5dadd..88672f0 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -1086,12 +1086,8 @@
             len = tf->name.len;
         }
 
-        reserve = len - r->uri.len;
-
         /* 16 bytes are preallocation */
-        reserve = reserve < 16 ? 16 : reserve + 16;
-
-        reserve += alias;
+        reserve = ngx_abs((ssize_t) (len - r->uri.len)) + alias + 16;
 
         if (reserve > allocated) {