runtime: clear sg.selectdone before saving in SudoG cache

Removes another dangling pointer that might
cause a memory leak in 1.4 or crash the GC in 1.5.

LGTM=rlh
R=golang-codereviews
CC=golang-codereviews, iant, khr, r, rlh
https://golang.org/cl/150520043

src/runtime/proc.go

@@ -174,6 +174,9 @@ func releaseSudog(s *sudog) {
 	if s.elem != nil {
 		gothrow("runtime: sudog with non-nil elem")
 	}
+	if s.selectdone != nil {
+		gothrow("runtime: sudog with non-nil selectdone")
+	}
 	gp := getg()
 	if gp.param != nil {
 		gothrow("runtime: releaseSudog with non-nil gp.param")

src/runtime/select.go

@@ -377,8 +377,14 @@ loop:
 	// iterating through the linked list they are in reverse order.
 	cas = nil
 	sglist = gp.waiting
-	// Clear all elem before unlinking from gp.waiting.
+	// Clear all selectdone and elem before unlinking from gp.waiting.
+	// They must be cleared before being put back into the sudog cache.
+	// Clear before unlinking, because if a stack copy happens after the unlink,
+	// they will not be updated, they will be left pointing to the old stack,
+	// which creates dangling pointers, which may be detected by the
+	// garbage collector.
 	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
+		sg1.selectdone = nil
 		sg1.elem = nil
 	}
 	gp.waiting = nil