detect: add more defensive checks for flow handling

Don't unconditionally deref f->alparser in detection through
DeStateFlowHasInspectableState(). In very rare cases it can
be NULL.

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/292
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/210