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

@@ -246,6 +246,7 @@ ngx_write_chain_to_file(ngx_file_t *file, ngx_chain_t *cl, off_t offset,
 
         file->sys_offset += n;
         file->offset += n;
+        offset += n;
         total += n;
 
     } while (cl);