Seems that digging a signal does not clear the IP position from the
advtrains database: by placing another signal in the same coordinates, the
IP results already set on the old pos.
I am not sure if this could cause some other issue, other than leaking
old/unnecessary info in the data.
Fixed.
In fact, it caused another issue: rails where an IP was assigned once
could not be removed, even with the signal no longer in place.