encoding/gob: panic: drop

[dvyukov]

Run the following program:
http://play.golang.org/p/EJ8Qd_lfDI

panic: drop
goroutine 1 [running]:
encoding/gob.catchError(0x1040e35c, 0x1)
    /usr/src/go/src/encoding/gob/error.go:38 +0xc0
encoding/gob.(*decBuffer).Drop(0x1040e330, 0x7f)
    /usr/src/go/src/encoding/gob/decode.go:55 +0xe0
encoding/gob.(*Decoder).ignoreInterface(0x1040e320, 0x104351c0)
    /usr/src/go/src/encoding/gob/decode.go:691 +0x260

Happens on both go14 (playground) and tip.