BUG/MEDIUM: stick-table: fix regression caused by a change in proxy struct

In commit 1b8e68e ("MEDIUM: stick-table: Stop handling stick-tables as
proxies."), the ->table member of proxy struct was replaced by a pointer
that is not always checked and in some situations can cause a segfault,
eg. during reload or while using "show table" on CLI socket.

No backport is needed.
diff --git a/src/haproxy.c b/src/haproxy.c
index 33f2e9d..f709224 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2367,7 +2367,8 @@
 
 		pool_destroy(p->req_cap_pool);
 		pool_destroy(p->rsp_cap_pool);
-		pool_destroy(p->table->pool);
+		if (p->table)
+			pool_destroy(p->table->pool);
 
 		p0 = p;
 		p = p->next;