cmd/gc: fix handling of for post-condition in -race mode

Fixes #8102.

LGTM=bradfitz, dvyukov
R=golang-codereviews, bradfitz, dvyukov
CC=golang-codereviews
https://golang.org/cl/100870046

src/cmd/gc/racewalk.c

@@ -182,7 +182,7 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 			// x, y := f() becomes BLOCK{CALL f, AS x [SP+0], AS y [SP+n]}
 			// We don't want to instrument between the statements because it will
 			// smash the results.
-			racewalknode(&n->list->n, &n->ninit, 0, 0);
+			racewalknode(&n->list->n, &n->list->n->ninit, 0, 0);
 			fini = nil;
 			racewalklist(n->list->next, &fini);
 			n->list = concat(n->list, fini);

src/pkg/runtime/race/race_test.go

@@ -155,3 +155,18 @@ func runTests() ([]byte, error) {
 	cmd.Env = append(cmd.Env, `GORACE="suppress_equal_stacks=0 suppress_equal_addresses=0 exitcode=0"`)
 	return cmd.CombinedOutput()
 }
+
+func TestIssue8102(t *testing.T) {
+	// If this compiles with -race, the test passes.
+	type S struct {
+		x interface{}
+		i int
+	}
+	c := make(chan int)
+	a := [2]*int{}
+	for ; ; c <- *a[S{}.i] {
+		if t != nil {
+			break
+		}
+	}
+}