Core: fixed ngx_write_chain_to_file() with IOV_MAX reached.

Catched by dav_chunked.t on Solaris.  In released versions this might
potentially result in corruption of complex protocol responses if they
were written to disk and there were more distinct buffers than IOV_MAX
in a single write.

src/os/unix/ngx_files.c

diff --git a/src/os/unix/ngx_files.c b/src/os/unix/ngx_files.c
index 65d2320..4fdf884 100644
--- a/src/os/unix/ngx_files.c
+++ b/src/os/unix/ngx_files.c
@@ -246,6 +246,7 @@
 
         file->sys_offset += n;
         file->offset += n;
+        offset += n;
         total += n;
 
     } while (cl);