runtime: clear stale values from G.param and SudoG.elem

This change was necessary on the dev.garbage branch
to keep the garbage collector from seeing pointers into
invalid heap areas.

On this default (Go 1.4) branch, the change removes
some possibility for memory leaks.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, iant, r, rlh
https://codereview.appspot.com/155760043