Avoid 'warning: assigned but unused variable' warn
Avoid 'warning: assigned but unused variable' warn

When $VERBOSE is true ruby will log error for unused variables, this
error can be suppressed by prefixing variable by '_', or having variable
named '_', which is canonical signal for 'unnecessary'

(refer to commit c9528f6a9e3157b349d3d81d20b9b4047ee9ff63 as well)

tierconnect/appcore2-ruby-mqtt,njh/ruby-mqtt,keshihoriuchi/ruby-mqtt