Bugfix: in ngx_http_upstream_dynamic_module

If fail_timeout is enabled and fallback action is specified
to either "stale" or "next", a segmentation fault would happen.

Signed-off-by: Paul Yang(InfoHunter) <paulyang.inf@gmail.com>
Merge pull request #499 from InfoHunter/dyn-dns

Bugfix: in ngx_http_upstream_dynamic_module