MDEV-34515: Fix a bogus debug assertion

purge_sys_t::stop_FTS(): Fix an incorrect debug assertion that
commit d58734d781 added.
The assertion would fail if there had been prior invocations of
purge_sys.stop_SYS() without purge_sys.resume_SYS().
The intention of the assertion is to check that number of pending
stop_FTS() stays below 65536.

storage/innobase/srv/srv0srv.cc

@@ -1299,7 +1299,7 @@ bool purge_sys_t::running()
 void purge_sys_t::stop_FTS()
 {
   ut_d(const auto paused=) m_FTS_paused.fetch_add(1);
-  ut_ad(paused < PAUSED_SYS);
+  ut_ad((paused + 1) & ~PAUSED_SYS);
   while (m_active.load(std::memory_order_acquire))
     std::this_thread::sleep_for(std::chrono::seconds(1));
 }