Merge 10.3 into 10.4, except for MDEV-20265

The MDEV-20265 commit e746f451d5
introduces DBUG_ASSERT(right_op == r_tbl) in
st_select_lex::add_cross_joined_table(), and that assertion would
fail in several tests that exercise joins. That commit was skipped
in this merge, and a separate fix of MDEV-20265 will be necessary in 10.4.

sql/sql_explain.cc

@@ -460,6 +460,8 @@ uint Explain_union::make_union_table_name(char *buf)
       break;
     default:
       DBUG_ASSERT(0);
+      type.str= NULL;
+      type.length= 0;
   }
   memcpy(buf, type.str, (len= (uint)type.length));