runtime: delay marking maps as writing until after first alg call

Fixes #19359

Change-Id: I196b47cf0471915b6dc63785e8542aa1876ff695
Reviewed-on: https://go-review.googlesource.com/37665
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

test/fixedbugs/issue19359.go

@@ -0,0 +1,37 @@
+// 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 "fmt"
+
+func set(m map[interface{}]interface{}, key interface{}) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("set failed: %v", r)
+		}
+	}()
+	m[key] = nil
+	return nil
+}
+
+func del(m map[interface{}]interface{}, key interface{}) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("del failed: %v", r)
+		}
+	}()
+	delete(m, key)
+	return nil
+}
+
+func main() {
+	m := make(map[interface{}]interface{})
+	set(m, []int{1, 2, 3})
+	set(m, "abc") // used to throw
+	del(m, []int{1, 2, 3})
+	del(m, "abc") // used to throw
+}