MDEV-3798: EXPLAIN UPDATE/DELETE

- Produce correct `key_len` when type=index.

mysql-test/r/explain_non_select.result

@@ -21,7 +21,7 @@ from t0 A, t0 B, t0 C;
 # This should use an index,  possible_keys=NULL because there is no WHERE
 explain delete from t1 order by a limit 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	a	NULL	NULL	2	
+1	SIMPLE	t1	index	NULL	a	5	NULL	2	
 # This should use range, possible_keys={a,b}
 explain delete from t1 where a<20 and b < 10;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

sql/sql_delete.cc

@@ -158,8 +158,11 @@ void Update_plan::save_explain_data_intern(Explain_query *query,
     if (index != MAX_KEY)
     {
       explain->key_str.append(table->key_info[index].name);
+      char buf[64];
+      size_t length;
+      length= longlong10_to_str(table->key_info[index].key_length, buf, 10) - buf;
+      explain->key_len_str.append(buf, length);
     }
-    // key_len stays NULL
   }
   explain->rows= scanned_rows;