MPLS Decoder, v5

Obsoletes #1160.

Use ENGINE_SET_INVALID_EVENT in the case of a too small MPLS header, and unknown payload type.

- PR build: https://buildbot.openinfosecfoundation.org/builders/jasonish/builds/15
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/jasonish-pcap/builds/15