cmd/gc: clear n->list of OFOR range loop after walk.

It contains the LHS of the range clause and gets
instrumented by racewalk, but it doesn't have any meaning.

Fixes #5446.

R=golang-dev, dvyukov, daniel.morsing, r
CC=golang-dev
https://golang.org/cl/9560044

src/cmd/gc/range.c

@@ -129,6 +129,9 @@ walkrange(Node *n)
 	v2 = N;
 	if(n->list->next)
 		v2 = n->list->next->n;
+	// n->list has no meaning anymore, clear it
+	// to avoid erroneous processing by racewalk.
+	n->list = nil;
 	hv2 = N;
 
 	if(v2 == N && t->etype == TARRAY) {

src/pkg/runtime/race/testdata/mop_test.go

@@ -267,6 +267,19 @@ func TestNoRaceRange(t *testing.T) {
 	close(ch)
 }
 
+func TestNoRaceRangeIssue5446(t *testing.T) {
+	ch := make(chan int, 3)
+	a := []int{1, 2, 3}
+	b := []int{4}
+	// used to insert a spurious instrumentation of a[i]
+	// and crash.
+	i := 1
+	for i, a[i] = range b {
+		ch <- i
+	}
+	close(ch)
+}
+
 func TestRaceRange(t *testing.T) {
 	const N = 2
 	var a [N]int