Client: fix the index bug in the Selector class

Signed-off-by: Jean-François Erdelyi <jean-francois.erdelyi.1@ulaval.ca>
Reviewed-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
Tested-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>