crash in main.views (and other view + PS tests)

When a view is merged, mark its select_lex as already optimized, otherwise
its where clause (which doesn't have to be a valid Item after merging)
might be accessed later. But don't do that for inserts (where a view cannot
be simply merged, if one later needs to insert into it).

sql/sql_derived.cc

@@ -465,6 +465,8 @@ bool mysql_derived_merge(THD *thd, LEX *lex, TABLE_LIST *derived)
     }
   }
 
+  if (!derived->merged_for_insert)
+    dt_select->first_cond_optimization= FALSE; // consider it optimized
 exit_merge:
   if (arena)
     thd->restore_active_arena(arena, &backup);