Fix cleaning_buffer_counter check (#21115)

Marlin/src/module/planner.cpp

   , feedRate_t fr_mm_s, const uint8_t extruder, const float &millimeters
 ) {
 
-  // If we are cleaning, do not accept queuing of movements
-  if (cleaning_buffer_counter) return false;
-
   // Wait for the next available block
   uint8_t next_buffer_head;
   block_t * const block = get_next_free_block(next_buffer_head);
 
+  // If we are cleaning, do not accept queuing of movements
+  // This must be after get_next_free_block() because it calls idle()
+  // where cleaning_buffer_counter can be changed
+  if (cleaning_buffer_counter) return false;
+
   // Fill the block with the specified movement
   if (!_populate_block(block, false, target
     #if HAS_POSITION_FLOAT