branches/zip: buf_pool_init(): Initialize buf_pool_zip_mutex before

acquiring buf_pool_mutex.  This avoids triggering the debug assertion
that was added in r2227.

buf/buf0buf.c

@@ -912,11 +912,10 @@ buf_pool_init(void)
 	/* 1. Initialize general fields
 	------------------------------- */
 	mutex_create(&buf_pool_mutex, SYNC_BUF_POOL);
+	mutex_create(&buf_pool_zip_mutex, SYNC_BUF_BLOCK);
 
 	buf_pool_mutex_enter();
 
-	mutex_create(&buf_pool_zip_mutex, SYNC_BUF_BLOCK);
-
 	buf_pool->n_chunks = 1;
 	buf_pool->chunks = chunk = mem_alloc(sizeof *chunk);