test/fixedbugs: add a test for 19201

This was cherry-picked to 1.8 as CL 38587, but on master issue was fixed
by CL 37661. Add still relevant part (test) and close issue, since test passes.

Fixes #19201

Change-Id: I6415792e2c465dc6d9bd6583ba1e54b107bcf5cc
Reviewed-on: https://go-review.googlesource.com/37376
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

test/fixedbugs/issue19201.go

+// run
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"encoding/binary"
+)
+
+var (
+	ch1 = make(chan int)
+	ch2 = make(chan int)
+
+	bin  = []byte("a\000\000\001")
+	want = binary.BigEndian.Uint32(bin)
+
+	c consumer = noopConsumer{}
+)
+
+type msg struct {
+	code uint32
+}
+
+type consumer interface {
+	consume(msg)
+}
+
+type noopConsumer struct{}
+
+func (noopConsumer) consume(msg) {}
+
+func init() {
+	close(ch1)
+}
+
+func main() {
+	var m msg
+	m.code = binary.BigEndian.Uint32(bin)
+
+	select {
+	case <-ch1:
+		c.consume(m)
+		if m.code != want {
+			// can not use m.code here, or it will work
+			panic("BigEndian read failed")
+		}
+	case <-ch2:
+	}
+}