stc15: check that a UID has been received

Found by fuzzing. In some cases it's possible that we end up without
a valid UID. Detect and workaround.

stcgal/protocols.py

@@ -1433,6 +1433,10 @@ class Stc15Protocol(Stc15AProtocol):
         if len(response) >= 8:
             self.uid = response[1:8]
 
+        # we should have a UID at this point
+        if not self.uid:
+            raise StcProtocolException("UID is missing")
+
     def program_flash(self, data):
         """Program the MCU's flash memory."""