gob: when decoding a string, allocate a string, not a []byte.

R=r, r2
CC=golang-dev
https://golang.org/cl/4430042

src/pkg/gob/decode.go

@@ -406,7 +406,7 @@ func decUint8Array(i *decInstr, state *decoderState, p unsafe.Pointer) {
 func decString(i *decInstr, state *decoderState, p unsafe.Pointer) {
 	if i.indir > 0 {
 		if *(*unsafe.Pointer)(p) == nil {
-			*(*unsafe.Pointer)(p) = unsafe.Pointer(new([]byte))
+			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(string))
 		}
 		p = *(*unsafe.Pointer)(p)
 	}