fix bug introduced in r1228

src/core/ngx_output_chain.c

diff --git a/src/core/ngx_output_chain.c b/src/core/ngx_output_chain.c
index 6bb463a..5e6773e 100644
--- a/src/core/ngx_output_chain.c
+++ b/src/core/ngx_output_chain.c
@@ -418,7 +418,7 @@
 
         src->file_pos += n;
 
-        if (src->pos == src->last) {
+        if (src->file_pos == src->file_last) {
             dst->flush = src->flush;
             dst->last_buf = src->last_buf;
         }