[gSender Issue#240] Resume job at line assumes ‘zMax + 10’ is valid

When resuming a job the gcode:start command assumes that ‘zMax + 10’ is a valid position. If the zMax of your job is within 10 units of your zLimit ‘resume from line’ will either hit your hard-limit if enabled, or cam-out and your Z axis will be off by ’10 – (zLimit – zMax)’ for the remainder of the job.

https://github.com/Sienci-Labs/gsender/blob/master/src/server/controllers/Grbl/GrblController.js#L1290