cmd/internal/obj/arm64: don't crash on trailing NOP

Fixes #20348

Change-Id: I831aeeee8e20d55b3e47dea67786e883b213cd58
Reviewed-on: https://go-review.googlesource.com/106457
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/arm64/asm_test.go

@@ -61,3 +61,21 @@ func gen(buf *bytes.Buffer) {
 	fmt.Fprintln(buf, "label:")
 	fmt.Fprintln(buf, "RET")
 }
+
+// Issue 20348.
+func TestNoRet(t *testing.T) {
+	dir, err := ioutil.TempDir("", "testnoret")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+	tmpfile := filepath.Join(dir, "x.s")
+	if err := ioutil.WriteFile(tmpfile, []byte("TEXT ·stub(SB),$0-0\nNOP\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	cmd := exec.Command(testenv.GoToolPath(t), "tool", "asm", "-o", filepath.Join(dir, "x.o"), tmpfile)
+	cmd.Env = []string{"GOARCH=arm64", "GOOS=linux"}
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Errorf("%v\n%s", err, out)
+	}
+}

src/cmd/internal/obj/arm64/obj7.go

@@ -476,9 +476,11 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 			break
 
 		case obj.ANOP:
+			if p.Link != nil {
 				q1 = p.Link
 				q.Link = q1 /* q is non-nop */
 				q1.Mark |= p.Mark
+			}
 			continue
 
 		case ABL,