encoding/hex: add Decode error test case

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/46880043

src/pkg/encoding/hex/hex_test.go

@@ -82,6 +82,7 @@ type errTest struct {
 var errTests = []errTest{
 	{"0", "encoding/hex: odd length hex string"},
 	{"0g", "encoding/hex: invalid byte: U+0067 'g'"},
+	{"00gg", "encoding/hex: invalid byte: U+0067 'g'"},
 	{"0\x01", "encoding/hex: invalid byte: U+0001"},
 }