encoding/base64: don't lose a byte of output when encountering trailing garbage

Fixes #7733

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews, nigeltao, r, rsc
https://golang.org/cl/88330044

src/pkg/encoding/base64/base64.go

@@ -250,7 +250,7 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 				}
 				if len(src) > 0 {
 					// trailing garbage
-					return n, false, CorruptInputError(olen - len(src))
+					err = CorruptInputError(olen - len(src))
 				}
 				dlen, end = j, true
 				break
@@ -277,7 +277,7 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 		n += dlen - 1
 	}
 
-	return n, end, nil
+	return n, end, err
 }
 
 // Decode decodes src using the encoding enc.  It writes at most

src/pkg/encoding/base64/base64_test.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"io"
 	"io/ioutil"
+	"reflect"
 	"strings"
 	"testing"
 	"time"
@@ -165,6 +166,7 @@ func TestDecodeCorrupt(t *testing.T) {
 		{"AAA=", -1},
 		{"AAAA", -1},
 		{"AAAAAA=", 7},
+		{"YWJjZA=====", 8},
 	}
 	for _, tc := range testCases {
 		dbuf := make([]byte, StdEncoding.DecodedLen(len(tc.input)))
@@ -329,3 +331,14 @@ bqbPb06551Y4
 		t.Error("Decoded results not equal")
 	}
 }
+
+func TestDecoderIssue7733(t *testing.T) {
+	s, err := StdEncoding.DecodeString("YWJjZA=====")
+	want := CorruptInputError(8)
+	if !reflect.DeepEqual(want, err) {
+		t.Errorf("Error = %v; want CorruptInputError(8)")
+	}
+	if string(s) != "abcd" {
+		t.Errorf("DecodeString = %q; want abcd", s)
+	}
+}