test_hs_ob: initialize pointers to NULL

This is an attempt to fix CID 1486280, where coverity warns us that,
if the tests failed, we might free() an uninitialized pointer.

src/test/test_hs_ob.c

@@ -174,6 +174,7 @@ test_get_subcredentials(void *arg)
   hs_subcredential_t *subcreds = NULL;
 
   (void) arg;
+  memset(&config, 0, sizeof(config));
 
   MOCK(networkstatus_get_live_consensus,
        mock_networkstatus_get_live_consensus);