dialects.mysql.connection-manager: Use `connection._protocol._ended` state on $validate

Change validation method, as discussed on #2381
I hate being nitpicky about the styling, but i also hate having to change it later.
Would you mind removing the whitespace so it becomes `[ 'disconnected', 'protocol_error'].indexOf(connection.state)`?

Otherwise i think it's good to go, good catch on finding that extra state @cusspvz