MDEV-30900 Crash on macOS due to zero-initialized buf_dblwr.write_cond

buf_dblwr_t::init(), buf_dblwr_t::close(): Cover also write_cond,
which was added in commit a55b951e
without explicit initialization. On GNU/Linux, PTHREAD_COND_INITIALIZER
is a zero-initializer. That is why the default zero initialization
happened to work on that platform.

storage/innobase/buf/buf0dblwr.cc

@@ -53,6 +53,7 @@ void buf_dblwr_t::init()
     active_slot= &slots[0];
     mysql_mutex_init(buf_dblwr_mutex_key, &mutex, nullptr);
     pthread_cond_init(&cond, nullptr);
+    pthread_cond_init(&write_cond, nullptr);
   }
 }
 
@@ -468,6 +469,7 @@ void buf_dblwr_t::close()
   ut_ad(!batch_running);
 
   pthread_cond_destroy(&cond);
+  pthread_cond_destroy(&write_cond);
   for (int i= 0; i < 2; i++)
   {
     aligned_free(slots[i].write_buf);