mime: fix panic parsing 'encoded-word'

https://go-review.googlesource.com/37812 says fix panic parsing.
Actually, it doesn't. so fix it.

Fixes #19416

Change-Id: Ie0c4241f10e5ebcbac20e184c2a7b13b22632eab
Reviewed-on: https://go-review.googlesource.com/37912Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/encodedword.go

@@ -208,15 +208,15 @@ func (d *WordDecoder) Decode(word string) (string, error) {
 	if len(charset) == 0 {
 		return "", errInvalidWord
 	}
+	if len(word) <= split+3 {
+		return "", errInvalidWord
+	}
 	encoding := word[split+1]
 	// the field after split must only be one byte
 	if word[split+2] != '?' {
 		return "", errInvalidWord
 	}
 	text := word[split+3:]
-	if len(text) == 0 {
-		return "", errInvalidWord
-	}
 
 	content, err := decode(encoding, text)
 	if err != nil {

src/mime/encodedword_test.go

@@ -90,6 +90,7 @@ func TestDecodeWord(t *testing.T) {
 		{"=?UTF-8?A?A?=", "", true},
 		{"=????=", "", true},
 		{"=?UTF-8?Q??=", "", true},
+		{"=?UTF-8???=", "", true},
 	}
 
 	for _, test := range tests {