MDEV-13352: Server crashes in st_join_table::remove_duplicates

join_tab->distinct=true means "Before doing record read with this
join_tab, call join_tab->remove_duplicates() to eliminate duplicates".
remove_duplicates() assumes that
- there is a temporary table $T with rows that are to be de-duplicated
- there is a previous join_tab (e.g. with join_tab->fields) which was
used to populate the temp.table $T.

When the query has "Impossible WHERE" and window function, then the above
conditions are not met (but we still might need a window function
computation step when the query has implicit grouping).

The fix is to not add remove_duplicates step if the select execution is
degenerate (and we'll have at most one row in the output anyway).

sql/sql_select.cc

@@ -2670,7 +2670,15 @@ bool JOIN::make_aggr_tables_info()
         curr_tab->having= having;
         having->update_used_tables();
       }
-      curr_tab->distinct= true;
+      /*
+        We only need DISTINCT operation if the join is not degenerate.
+        If it is, we must not request DISTINCT processing, because
+        remove_duplicates() assumes there is a preceding computation step (and
+        in the degenerate join, there's none)
+      */
+      if (top_join_tab_count)
+        curr_tab->distinct= true;
+
       having= NULL;
       select_distinct= false;
     }