runtime: zero a few more dead pointers.

In channels, zeroing of gp.waiting is missed on a closed channel panic.
m.morebuf.g is not zeroed.

I don't expect the latter causes any problems, but just in case.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/151610043