[EN-1413] Fixed segfault, fixed fixture

Author: asalynskii

dxfeed/core/DXFeedPy.pyx

@@ -393,6 +393,7 @@ def dxf_close_subscription(SubscriptionClass sc):
     if sc.subscription:
         clib.dxf_close_subscription(sc.subscription)
         sc.subscription = NULL
+        sc.con_sub_list_ptr[0][sc.subscription_order] = NULL
 
 def dxf_get_current_connection_status(ConnectionClass cc, return_str: bool=True):
     """

tests/test_dxfeedpy.py

@@ -1,14 +1,13 @@
 import dxfeed as dx
 import pytest
 
-
 class ValueStorage(object):  # config
     demo_address = 'demo.dxfeed.com:7300'
     event_types = ['Trade', 'Quote', 'Summary', 'Profile', 'Order', 'TimeAndSale', 'Candle', 'TradeETH', 'SpreadOrder',
                    'Greeks', 'TheoPrice', 'Underlying', 'Series', 'Configuration']
 
 
-@pytest.fixture
+@pytest.fixture(scope='function')
 def connection():
     # Setup
     con = dx.dxf_create_connection(ValueStorage.demo_address)