detect: fix tx iterator logic in detect

detect: fix tx iterator logic in detect

The 'tx_id' variable was used to be passed into the IterFunc as a
minumum tx to return. The IterFunc could then return either the tx
for that id, or a later one if that turned out to be the first available
tx.

The tx_id however, was still used for some things as if it was the
current tx id. Most importantly for setting the tx id for alert
ammending. So this could lead to alerts with missing or wrong
applayer records.