Avoid assert()

By default, CMAKE_BUILD_TYPE RelWithDebInfo or Release implies -DNDEBUG,
which disables the assert() macro. MariaDB is deviating from that.
Let us be explicit to use assert() only in debug builds.

This fixes up 1b8358d

storage/innobase/include/trx0trx.h

@@ -618,14 +618,14 @@ struct trx_t : ilist_node<>
   {
     ut_ad(!mutex_is_owner());
     mutex.wr_lock();
-    assert(!mutex_owner.exchange(pthread_self(),
-                                 std::memory_order_relaxed));
+    ut_d(assert(!mutex_owner.exchange(pthread_self(),
+                                      std::memory_order_relaxed)));
   }
   /** Release the mutex */
   void mutex_unlock()
   {
-    assert(mutex_owner.exchange(0, std::memory_order_relaxed) ==
-           pthread_self());
+    ut_d(assert(mutex_owner.exchange(0, std::memory_order_relaxed) ==
+                pthread_self()));
     mutex.wr_unlock();
   }
 #ifndef SUX_LOCK_GENERIC