Give WS_Reservation() the same "always return non-NULL or assert"
semantics WS_Front() had because literally every single caller which
uses the ->f pointer fails to check for NULL.

Introduce a new internal WS_IsReserved() for use in the asserts
which check if a reservation is active, but which doesn't otherwise
care for the ->f pointer, and the single instance of non-WS code
which (possibly) legitimatly does variant processing depending on
the reservation state.