encoding/base64: fix panic when input len is not a multiple of 4

Fixes #3442.

R=for.go.yong, dsymonds, sougou, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/5975052

src/pkg/encoding/base64/base64.go

@@ -230,7 +230,12 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 			if in == '=' && j >= 2 && len(src) < 4 {
 				// We've reached the end and there's
 				// padding
+				if len(src) == 0 && j == 2 {
+					// not enough padding
+					return n, false, CorruptInputError(len(osrc))
+				}
 				if len(src) > 0 && src[0] != '=' {
+					// incorrect padding
 					return n, false, CorruptInputError(len(osrc) - len(src) - 1)
 				}
 				dlen = j

src/pkg/encoding/base64/base64_test.go

@@ -151,6 +151,9 @@ func TestDecodeCorrupt(t *testing.T) {
 		{"AAA=AAAA", 3},
 		{"AAAAA", 4},
 		{"AAAAAA", 4},
+		{"A=", 1},
+		{"AA=", 3},
+		{"AAAAAA=", 7},
 	}
 
 	for _, e := range examples {