encoding/ascii85: fix panic caused by special case

Special case for encoding 4 zeros as 'z' didn't
update source slice, causing 'index out of bounds'
panic in destination slice.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5970078

src/pkg/encoding/ascii85/ascii85.go

@@ -57,6 +57,7 @@ func Encode(dst, src []byte) int {
 		if v == 0 && len(src) >= 4 {
 			dst[0] = 'z'
 			dst = dst[1:]
+			src = src[4:]
 			n++
 			continue
 		}

src/pkg/encoding/ascii85/ascii85_test.go

@@ -28,6 +28,11 @@ var pairs = []testpair{
 			"l(DId<j@<?3r@:F%a+D58'ATD4$Bl@l3De:,-DJs`8ARoFb/0JMK@qB4^F!,R<AKZ&-DfTqBG%G\n" +
 			">uD.RTpAKYo'+CT/5+Cei#DII?(E,9)oF*2M7/c\n",
 	},
+	// Special case when shortening !!!!! to z.
+	{
+		"\000\000\000\000",
+		"z",
+	},
 }
 
 var bigtest = pairs[len(pairs)-1]