If '[stepper_x]' and/or '[stepper_y]' have a 'position_min' that is
non-zero, and '[safe_z_home] home_xy_position' is '0,0'; then the 'G28'
command will _not_ move to '0,0'; but stay at 'position_min' during the
Z endstop test.
This fix corrects this issue.
Signed-off-by: Jason S. McMullan <jason.mcmullan@gmail.com>