tap_manager: don't try to remove the end
tap_manager: don't try to remove the end

The iterator pos must be valid and dereferenceable. Thus the end()
iterator (which is valid, but is not dereferencable) cannot be used as a
value for pos.

From: https://en.cppreference.com/w/cpp/container/map/erase

fixes 63ed6733

bisdn/basebox,bisdn/basebox,bisdn/basebox,bisdn/basebox,toanju/basebox,toanju/basebox