native-fetcher: fix a crasher, and set a timeout

Handle the case where ngx_event_connect_peer didn't return NGX_OK.
Also replace a TODO for adding a timer with actually adding the
timer.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/463
Merge pull request #465 from pagespeed/oschaaf-native-fetcher-crash

native-fetcher: fix a crasher, and set a timeout