runtime: clear f, arg to avoid leak in timerproc

runtime: clear f, arg to avoid leak in timerproc

I have seen this cause leaks where not all objects in a sync.Pool
would be reclaimed during the sync package tests.
I found it while debugging the '0 of 100 finalized' failure we are
seeing on arm, but it seems not to be the root cause for that one.

LGTM=dave, dvyukov
R=golang-codereviews, dave, dvyukov
CC=golang-codereviews
https://golang.org/cl/64920044