-b and -c options are not being passed

In attempting to pass both a -b and -c variable to the script is causing an error and exiting the script.  I've been testing on `system-rescue-cd 12.01` and `disk-burnin.sh` v1.1.0.

I suspect that the variables are becoming `readonly` and is fixing the default rather than updating with the passed options.

I code very little and only basic scripts in bash so I could be completely off base.