sensu-cli.bash throws errors during tab completion

`sensu-cli` throws errors when tab-completing commands. The error is `-bash: COMP_WORDS: bad array subscript` and appears to be caused by https://github.com/agent462/sensu-cli/blob/47cee1b569/contrib/sensu-cli.bash#L27-L28, which attempt to reference array elements less than 0.
