Relax an assertion in start_selection()

It asserted that the previous count was always nonnegative, but DISCONNECTED is
a valid value for it to see. In order to continue to remember to store
DISCONNECTED after DISCONNECTED was seen, I also added a helper method.

Closes #12226
auto merge of #12227 : alexcrichton/rust/chan-select-bug, r=kballard

It asserted that the previous count was always nonnegative, but DISCONNECTED is
a valid value for it to see. In order to continue to remember to store
DISCONNECTED after DISCONNECTED was seen, I also added a helper method.

Closes #12226