[BUG] counters: fix segfault on missing counters for a listener

If a frontend does not set 'option socket-stats', a 'clear counters'
on the stats socket could segfault because li->counters is NULL. The
correct fix is to check for NULL before as this is a valid situation.

src/dumpstats.c

@@ -317,7 +317,8 @@ int stats_sock_parse_request(struct stream_interface *si, char *line)
 					memset(&sv->counters, 0, sizeof(sv->counters));
 
 				for (li = px->listen; li; li = li->next)
-					memset(li->counters, 0, sizeof(*li->counters));
+					if (li->counters)
+						memset(li->counters, 0, sizeof(*li->counters));
 			}
 
 			return 1;