cmd/gc: fix crash during escape analysis

Fixes #10441

Compiler crashed as:

panic: runtime error: invalid memory address or nil pointer dereference
goroutine 1 [running]:
cmd/internal/gc.haspointers(0x0, 0xffffffffffffffff)
    src/cmd/internal/gc/reflect.go:623 +0x1f
cmd/internal/gc.escwalk(0x20c82dc00, 0xffffffffffffffff, 0x20c82dda0, 0x20c835520)
    src/cmd/internal/gc/esc.go:1291 +0x14aa
cmd/internal/gc.escwalk(0x20c82dc00, 0xffffffffffffffff, 0x20c82dda0, 0x20beac340)
    src/cmd/internal/gc/esc.go:1386 +0x836

while evaluating haspointers of the fake OADDR node here:

	leaks = level <= 0 && dst.Escloopdepth < src.Escloopdepth || level < 0 && dst == &e.funcParam && haspointers(src.Type)

Change-Id: I13c23fced52e8fcd0ae1df81b374df8632eb7790
Reviewed-on: https://go-review.googlesource.com/8932Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/gc/esc.go

@@ -885,6 +885,7 @@ func escassign(e *EscState, dst *Node, src *Node) {
 		a := Nod(OADDR, src, nil)
 		a.Lineno = src.Lineno
 		a.Escloopdepth = src.Escloopdepth
+		a.Type = Ptrto(src.Type)
 		escflows(e, dst, a)
 
 		// Flowing multiple returns to a single dst happens when

test/fixedbugs/issue10441.go

+// build
+
+// Copyright 2015 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 p
+
+func bar() {
+	f := func() {}
+	foo(&f)
+}
+
+func foo(f *func()) func() {
+	defer func() {}() // prevent inlining of foo
+	return *f
+}