fix AttributeError when executing close

When self.device is already None an AttributeError will occur.
This is relevant to #189.

src/escpos/printer.py

@@ -211,6 +211,7 @@ class Network(Escpos):
 
     def close(self):
         """ Close TCP connection """
+        if self.device is not None:
             self.device.shutdown(socket.SHUT_RDWR)
             self.device.close()
 
@@ -263,6 +264,7 @@ class File(Escpos):
 
     def close(self):
         """ Close system file """
+        if self.device is not None:
             self.device.flush()
             self.device.close()