MDEV-20097 potential use-after-free

row_merge_read_clustered_index(): fix one more place with buf and merge_buf[i]

storage/innobase/row/row0merge.cc

@@ -2431,6 +2431,7 @@ row_merge_read_clustered_index(
 							BTR_SEARCH_LEAF, &pcur,
 							&mtr);
 						buf = row_merge_buf_empty(buf);
+						merge_buf[i] = buf;
 						/* Restart the outer loop on the
 						record. We did not insert it
 						into any index yet. */