Fix for LA (#9964)

Missed in the original LA 1.5 PR: eISR has to use a local copy of
current_block->use_advance_lead because it might still run wenn the last
block has been set to NULL.

Marlin/src/module/stepper.cpp

@@ -776,7 +776,7 @@ void Stepper::isr() {
       #define STOP_E_PULSE(INDEX) do { if (e_steps) { E## INDEX ##_STEP_WRITE(INVERT_E_STEP_PIN); e_steps < 0 ? ++e_steps : --e_steps; } }while(0)
     #endif
 
-    if (current_block->use_advance_lead) {
+    if (use_advance_lead) {
       if (step_events_completed > LA_decelerate_after && current_adv_steps > final_adv_steps) {
         e_steps--;
         current_adv_steps--;