Fix possible race condition in printcore.startprint

printrun/printcore.py

@@ -357,10 +357,10 @@ class printcore():
         """
         if self.printing or not self.online or not self.printer:
             return False
-        self.printing = True
+        self.queueindex = startindex
         self.mainqueue = gcode
+        self.printing = True
         self.lineno = 0
-        self.queueindex = startindex
         self.resendfrom = -1
         self._send("M110", -1, True)
         if not gcode or not gcode.lines: