Reexecution of prepared "ANALYZE TABLE merge_table, table" may miss to
reinitialize "table" for subsequent execution and trigger assertion failure.

This happens because MERGE engine may adjust table->next_global chain, which
gets cleared by close_thread_tables()/ha_myisammrg::detach_children() later.
Since reinitilization iterates next_global chain, it won't see tables following
merge table.

Fixed by appending saved next_global chain after merge children.