This assert was triggered if DELETE was done on a view that
referenced another view which in turn (directly or indirectly)
referenced more than one table.

Delete from a view referencing more than one table (a join view)
is not supported and is supposed to give ER_VIEW_DELETE_MERGE_VIEW
error. Before this error was reported from the multi table 
delete code, an assert verified that the view from the DELETE statement
had more than one underlying table. However, this assert did not take
into account that the view could refer to another view which in turn
referenced the actual tables.

This patch fixes the problem by adjusting the assert to take this
possibility into account. This problem was only noticeable on debug
builds of the server. On release builds, ER_VIEW_DELETE_MERGE_VIEW
was correctly reported.

Test case added to delete.test.