BUG#36197: flush tables (or little table cache) can cause crash on slave

When flushing tables, there were a slight chance that the flush was occuring
between processing of two table map events. Since the tables are opened
one by one, it might result in that the tables were not valid and that sub-
sequent locking of tables would cause the slave to crash.

The problem is solved by opening and locking all tables at once using
simple_open_n_lock_tables(). Also, the patch contain a change to open_tables()
so that pre-locking only takes place when the trg_event_map is not zero, which
was not the case before (this caused the lock to be placed in thd->locked_tables
instead of thd->lock since the assumption was that triggers would be called
later and therefore the tables should be pre-locked).

sql/sql_base.cc

@@ -4367,6 +4367,11 @@ bool fix_merge_after_open(TABLE_LIST *old_child_list, TABLE_LIST **old_last,
     prelocking it won't do such precaching and will simply reuse table list
     which is already built.
 
+    If any table has a trigger and start->trg_event_map is non-zero
+    the final lock will end up in thd->locked_tables, otherwise, the
+    lock will be placed in thd->lock. See also comments in
+    st_lex::set_trg_event_type_for_tables().
+
   RETURN
     0  - OK
     -1 - error
@@ -4579,7 +4584,7 @@ int open_tables(THD *thd, TABLE_LIST **start, uint *counter, uint flags)
         process its triggers since they never will be activated.
       */
       if (!thd->prelocked_mode && !thd->lex->requires_prelocking() &&
-          tables->table->triggers &&
+          tables->trg_event_map && tables->table->triggers &&
           tables->lock_type >= TL_WRITE_ALLOW_WRITE)
       {
         if (!query_tables_last_own)