cmd/gc: do not crash on struct with _ field

Fixes #3607.

R=ken2
CC=golang-dev
https://golang.org/cl/6296052

src/cmd/gc/subr.c

@@ -2684,7 +2684,7 @@ genhash(Sym *sym, Type *t)
 		first = T;
 		for(t1=t->type;; t1=t1->down) {
 			if(t1 != T && (isblanksym(t1->sym) || algtype1(t1->type, nil) == AMEM)) {
-				if(first == T)
+				if(first == T && !isblanksym(t1->sym))
 					first = t1;
 				continue;
 			}
@@ -2901,7 +2901,7 @@ geneq(Sym *sym, Type *t)
 		first = T;
 		for(t1=t->type;; t1=t1->down) {
 			if(t1 != T && (isblanksym(t1->sym) || algtype1(t1->type, nil) == AMEM)) {
-				if(first == T)
+				if(first == T && !isblanksym(t1->sym))
 					first = t1;
 				continue;
 			}

test/fixedbugs/bug442.go

+// run
+
+// Copyright 2012 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.
+
+// Used to crash generating hash and == functions for struct
+// with leading _ field.  Issue 3607.
+
+package main
+
+type T struct {
+	_ int
+	X interface{}
+	_ string
+	Y float64
+}
+
+func main() {
+	m := map[T]int{}
+	m[T{X: 1, Y: 2}] = 1
+	m[T{X: 2, Y: 3}] = 2
+	m[T{X: 1, Y: 2}] = 3  // overwrites first entry
+	if len(m) != 2 {
+		println("BUG")
+	}
+}