Fix crash when start_install_environment fails (issue #103)

When start_install_environment fails (returns <=0) the code immediatelly
proceeds to handle_transfer_env_done where current_kids is decremented
after asserting it's >0. However, current_kids is only incremented if
start_install_environment returned >0, creating an inconsistency that
leads to a crash. The simple fix is to always increment current_kids.