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://golang.org/cl/155760043

src/runtime/proc.go

@@ -148,6 +148,9 @@ func acquireSudog() *sudog {
 	c := gomcache()
 	s := c.sudogcache
 	if s != nil {
+		if s.elem != nil {
+			gothrow("acquireSudog: found s.elem != nil in cache")
+		}
 		c.sudogcache = s.next
 		return s
 	}
@@ -168,6 +171,13 @@ func acquireSudog() *sudog {
 
 //go:nosplit
 func releaseSudog(s *sudog) {
+	if s.elem != nil {
+		gothrow("runtime: sudog with non-nil elem")
+	}
+	gp := getg()
+	if gp.param != nil {
+		gothrow("runtime: releaseSudog with non-nil gp.param")
+	}
 	c := gomcache()
 	s.next = c.sudogcache
 	c.sudogcache = s