filtered replication: panic on source tablet if charset is wrong

[michael-berlin]

Stacktrace observed by @sjmudd

binlog streamer is about to return this error: https://github.com/youtube/vitess/blob/master/go/vt/binlog/binlog_streamer.go#L201

But then it panics here due to a nil value: https://github.com/youtube/vitess/blob/master/go/vt/mysqlctl/slave_connection.go#L349

Because of that, the destination just shows this on the /debug/status page: Error received from Stream rpc error: code = Unknown desc = uncaught panic: runtime error: invalid memory address or nil pointer dereference

Alain, this is probably an easy fix?