codec: json: fix base64 decoding for binary data

DecodedLen is the maximum length. This means that we MUST check
the actual decoded length after decoding, and possibly truncate
the returned slice appropriately.

Fixes #70

codec/json.go

@@ -733,7 +733,14 @@ func (d *jsonDecDriver) DecodeBytes(bs []byte, isstring, zerocopy bool) (bsOut [
 	} else {
 		bsOut = make([]byte, slen)
 	}
-	base64.StdEncoding.Decode(bsOut, bs0)
+	slen2, err := base64.StdEncoding.Decode(bsOut, bs0)
+	if err != nil {
+		d.d.errorf("json: error decoding base64 binary '%s': %v", bs0, err)
+		return nil
+	}
+	if slen != slen2 {
+		bsOut = bsOut[:slen2]
+	}
 	return
 }