Less indentation in Stepper::isr

Marlin/stepper.cpp

@@ -357,11 +357,10 @@ void Stepper::isr() {
     }
     else {
       OCR1A = 2000; // 1kHz.
+      return;
     }
   }
 
-  if (current_block) {
-
   // Update endstops state, if enabled
   if (endstops.enabled
     #if HAS_BED_PROBE
@@ -667,7 +666,6 @@ void Stepper::isr() {
     current_block = NULL;
     planner.discard_current_block();
   }
-  }
 }
 
 #if ENABLED(ADVANCE) || ENABLED(LIN_ADVANCE)