If a prepared statement used both a MyISAMMRG table and a stored 
function or trigger, execution could fail with "No such table"
error or crash. 
The error would come from a failure of the MyISAMMRG engine
to meet the expectations of the prelocking algorithm, 
in particular maintain lex->query_tables_own_last pointer
in sync with lex->query_tables_last pointer/the contents
of lex->query_tables. When adding merge children, the merge
engine would extend the table list. Then, when adding 
prelocked tables, the prelocking algorithm would use a pointer
to the last merge child to assign to lex->query_tables_own_last.
Then, when merge children were removed at the end of
open_tables(), lex->query_tables_own_last
was not updated, and kept pointing
to a removed merge child.

The fix ensures that query_tables_own_last is always in
sync with lex->query_tables_last.

This is a regression introduced by WL#4144 and present only
in next-4284 tree and 6.0.