Fix error in DetectFlowintParse(). Assigning to both values of a union

Fix a bug in flowint parsing that was always assigning to one part of a union (target.value) whlie sometimes assigning to the other part (tarvalue). This only showed up in the unit tests after cleaning up flowint matching to not write the tvar.idx part of the context, since the match functions should not write any of the context.

Since Targetvar.idx is then never used, remove it, which saves 8 bytes from DetectFlowintData. It also makes the code more clear.

Passes PR scripts:
- PR build: https://buildbot.openinfosecfoundation.org/builders/ken-tilera/builds/50
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/ken-tilera-pcap/builds/56