encoding/base64: do not slice past output unnecessarily

Base64-encoding 32 bytes results in a 44-byte string.
While in general a 44-byte string might decode to 33 bytes,
if you take a 44-byte string that actually only encodes 32 bytes,
and you try to decode it into 32 bytes, that should succeed.
Instead it fails trying to do a useless dst[33:] slice operation.
Delete that slice operation.

Noticed while preparing CL 156322.

Change-Id: I8024bf28a65e2638675b980732b2ff91c66c62cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/164628Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/base64/base64.go

@@ -282,7 +282,7 @@ func (e CorruptInputError) Error() string {
 func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err error) {
 	// Decode quantum using the base64 alphabet
 	var dbuf [4]byte
-	dinc, dlen := 3, 4
+	dlen := 4
 
 	for j := 0; j < len(dbuf); j++ {
 		if len(src) == si {
@@ -292,7 +292,7 @@ func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err err
 			case j == 1, enc.padChar != NoPadding:
 				return si, 0, CorruptInputError(si - j)
 			}
-			dinc, dlen = j-1, j
+			dlen = j
 			break
 		}
 		in := src[si]
@@ -344,7 +344,7 @@ func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err err
 			// trailing garbage
 			err = CorruptInputError(si)
 		}
-		dinc, dlen = 3, j
+		dlen = j
 		break
 	}
 
@@ -369,7 +369,6 @@ func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err err
 			return si, 0, CorruptInputError(si - 2)
 		}
 	}
-	dst = dst[dinc:]
 
 	return si, dlen - 1, err
 }

src/encoding/base64/base64_test.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"io/ioutil"
 	"reflect"
+	"runtime/debug"
 	"strings"
 	"testing"
 	"time"
@@ -247,6 +248,20 @@ func TestDecodeCorrupt(t *testing.T) {
 	}
 }
 
+func TestDecodeBounds(t *testing.T) {
+	var buf [32]byte
+	s := StdEncoding.EncodeToString(buf[:])
+	defer func() {
+		if err := recover(); err != nil {
+			t.Fatalf("Decode panicked unexpectedly: %v\n%s", err, debug.Stack())
+		}
+	}()
+	n, err := StdEncoding.Decode(buf[:], []byte(s))
+	if n != len(buf) || err != nil {
+		t.Fatalf("StdEncoding.Decode = %d, %v, want %d, nil", n, err, len(buf))
+	}
+}
+
 func TestEncodedLen(t *testing.T) {
 	for _, tt := range []struct {
 		enc  *Encoding