interp: don't set an invalid sequence number
interp: don't set an invalid sequence number

When executing a queue-buster such as M66, the call chain
    emcTaskIssueCommand > emcTaskPlanExecute > Interp::execute
sets the sequence number from something valid to 0.  In this case,
command is NULL.  Skip setting sequence number in this specific case.

Signed-off-by: Jeff Epler <623a871f32baaca066adfe0cac08d7aa7d34027d@unpythonic.net>

bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror