Fix live reload segv when startup isn't complete

If a live reload signal was given before the engine was fully started
up (e.g. pcap file thread waiting for a disk to spin up), a segv could
occur.

This patch only enables live reloads after the threads have been
started up completely.

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/320
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/235