Index search with a bad external continuation value crashes API server

[mcesaro]

Passing an invalid continuation term to an index search crashes the API server.
The riak_index:decode_continuation/2 call will fail if the passed term can't be converted to binary:

%% @doc decode a continuation received from the outside world.
-spec decode_continuation(continuation() | undefined) -> last_result() | undefined.
decode_continuation(undefined) ->
    undefined;
decode_continuation(Bin) ->
    binary_to_term(base64:decode(Bin)).

If the passed term is invalid, the invocation will raise an exception:

* exception error: bad argument
     in function  binary_to_term/1
        called as binary_to_term(<<>>)
        *** argument 1: invalid external representation of a term
```

Trapping it inside a `try ... catch`  block to provide a sensible default would make  `apply_continuation/2` to gracefully fail.

[martinsumner]

For the HTTP API it is better to validate inputs in the riak_kv_wm_index:malformed_request/2 callback, and then a correct 4xx response code will be returned if it is invalid.

So we probably should decode the continuation within the API, with a try .. catch clause as you suggest, and pass only correctly decoded continuations to the riak_index:to_index_query/2 function.

It would be good to then confirm this with an addition to the riak_test test for continuation.

[mcesaro]

Please note that I triggered the crash using the erlang grpc client and the continuation value I passed was an empty binary. So the error return value semantic might be bad_continuation_type and bad_continuation_value to handle both cases.