ModemATCommandLink: fix AttributeError exception in __del__()

self._sl will not be assigned if serial.Serial() fails.

Change-Id: I179a7efd92eaa3469a17b6ef252b3949c44ea6ea

pySim/transport/modem_atcmd.py

@@ -43,7 +43,8 @@ class ModemATCommandLink(LinkBase):
 		self.reset_card()
 
 	def __del__(self):
-		self._sl.close()
+		if hasattr(self, '_sl'):
+			self._sl.close()
 
 	def send_at_cmd(self, cmd, timeout=0.2, patience=0.002):
 		# Convert from string to bytes, if needed