Prevent null pointer crash in Endstops::update

Thanks to Evgeny Kotsuba!

Marlin/endstops.cpp

@@ -421,6 +421,8 @@ void Endstops::update() {
   /**
    * Check and update endstops according to conditions
    */
+  if (stepper.current_block) {
+
     if (X_MOVE_TEST) {
       if (stepper.motor_direction(X_AXIS_HEAD)) { // -direction
         #if HAS_X_MIN
@@ -535,6 +537,8 @@ void Endstops::update() {
       }
     }
 
+  } // stepper.current_block
+
   old_endstop_bits = current_endstop_bits;
 
 } // Endstops::update()