Don't lose pointer to first nonempty buf in ngx_*_sendfile_chain().

Don't lose pointer to first nonempty buf in ngx_*_sendfile_chain().

In ngx_*_sendfile_chain() when calculating pointer to a first
non-zero sized buf, use "in" as iterator.  This fixes processing
of zero sized buf(s) after EINTR.  Otherwise function can return
zero sized buf to caller, and later ngx_http_write_filter()
logs warning.