OVSDriver: check for zero ttl when processing OF_ACTION_NICIRA_DEC_TTL

This could previously underflow if we received a packet with ttl == 0.

Partial fix for XE-51.
Merge into master from pull request #33:
OVSDriver: check for zero ttl when processing OF_ACTION_NICIRA_DEC_TTL (https://github.com/floodlight/ivs/pull/33)