Fix creation of invalid negative transport ids by cast uint32 -> int

Port of KRandom::random to QRandomGenerator in
b5572156 missed that generate() uses all
bits of the uint32, and the compiler does not warn when casting uint to int

Current code assumes only positive ids to be valid, so some checks will
assume the transport is not correctly configured.

Also set 1 as lowest minimal value, as a 0 value will be discarded anyway.