serial: do not try to close non existing serial connection

The SerialSimLink only has an _sl member if serial initalization was
successfull. If we close a serial connection, check if we even have the
_sl member. Otherwise move on silently.

Change-Id: Ic3f3f5e50d780f424da7d0be5733d7167bb7159c

pySim/transport/serial.py

@@ -52,7 +52,8 @@ class SerialSimLink(LinkBase):
 		self._atr = None
 
 	def __del__(self):
-		self._sl.close()
+		if (hasattr(self, "_sl")):
+			self._sl.close()
 
 	def wait_for_card(self, timeout=None, newcardonly=False):
 		# Direct try