Merge pull request #4688 from AnHardt/crittical_section

Repair MarlinSerial with TX-buffer

Marlin/MarlinSerial.cpp

@@ -244,10 +244,11 @@ void MarlinSerial::flush(void) {
     }
 
     tx_buffer.buffer[tx_buffer.head] = c;
-    CRITICAL_SECTION_START;
+    { CRITICAL_SECTION_START;
         tx_buffer.head = i;
         SBI(M_UCSRxB, M_UDRIEx);
       CRITICAL_SECTION_END;
+    }
     return;
   }