protobuf/jsonpb: fix panic when decoding non-string bytes
```
exec cue eval schema.cue json+pb: data.json
-- schema.cue --
t: bytes @protobuf(1, bytes)
-- data.json --
{"t": 42}
```
This would panic because of a logic error in jsonpb/decoder.go
Signed-off-by: Matthew Sackman <matthew@cue.works>
Change-Id: Ie9c0bf1670a01f3223fb85d354a36ff1dfacedda
Reviewed-on: https://cue.gerrithub.io/c/cue-lang/cue/+/1234625
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>