Continue from the line following the M999 (PR#60)

If the printer is going to accept the M999,
then it should continue from that line number.

Marlin/Marlin_main.cpp

 inline void gcode_M999() {
   Running = true;
   lcd_reset_alert_level();
-  gcode_LastN = Stopped_gcode_LastN;
+  // gcode_LastN = Stopped_gcode_LastN;
   FlushSerialRequestResend();
 }