Keep extruders cool at the beginning of the print if the won't be used for longer than

the re-heat time.
issue: MatterHackers/MCCentral#5500
Inactive hotend cooldown does not apply at beginning of print

MatterControl.Printing/GCode/GCodeMemoryFile.cs

@@ -392,7 +392,12 @@ namespace MatterControl.Printing
 
 		public override PrinterMachineInstruction Instruction(int index)
 		{
-			return gCodeCommandQueue[index];
+			if (index < gCodeCommandQueue.Count)
+			{
+				return gCodeCommandQueue[index];
+			}
+
+			return new PrinterMachineInstruction("");
 		}
 
 		public override bool IsExtruding(int instructionIndexToCheck)