fix a crash in innodb.innodb-wl5522-zip,xtradb

dereferencing of the uninitialized pointer bpade->slot
(when compiled without UNIV_DEBUG)

storage/innobase/buf/buf0buf.cc

@@ -3825,6 +3825,8 @@ err_exit:
 		page_zip_set_size(&bpage->zip, zip_size);
 		bpage->zip.data = (page_zip_t*) data;
 
+		bpage->slot = NULL;
+
 		mutex_enter(&buf_pool->zip_mutex);
 		UNIV_MEM_DESC(bpage->zip.data,
 			      page_zip_get_size(&bpage->zip));

storage/xtradb/buf/buf0buf.cc

@@ -3895,6 +3895,8 @@ err_exit:
 		page_zip_set_size(&bpage->zip, zip_size);
 		bpage->zip.data = (page_zip_t*) data;
 
+		bpage->slot = NULL;
+
 		mutex_enter(&buf_pool->zip_mutex);
 		UNIV_MEM_DESC(bpage->zip.data,
 			      page_zip_get_size(&bpage->zip));