core: timer_enter_waiting(): refactor `base` local variable
We have two variables `b` and `base`. `b` is declared within limited
scope; `base` is declared at the top of the function. However `base`
is actually only used within a scope which is exclusive of `b`. Clarify
by moving `base` inside the limited scope as well.
(Also `base` doesn't need initializing any more than `b` does. The
declaration of `base` is now immediately followed by a case analysis of
`v->base`, which serves almost exclusively to determine the value of
`base`).