cmd/gc: compute size of keys & values before making map bucket

Fixes #7547

LGTM=iant
R=iant, khr
CC=golang-codereviews
https://golang.org/cl/84470046

src/cmd/gc/reflect.c

@@ -125,6 +125,8 @@ mapbucket(Type *t)
 
 	keytype = t->down;
 	valtype = t->type;
+	dowidth(keytype);
+	dowidth(valtype);
 	if(keytype->width > MAXKEYSIZE)
 		keytype = ptrto(keytype);
 	if(valtype->width > MAXVALSIZE)

test/fixedbugs/issue7547.go

+// compile
+
+// Copyright 2014 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
+
+func f() map[string]interface{} {
+	var p *map[string]map[string]interface{}
+	_ = p
+	return nil
+}
+
+func main() {
+	f()
+}