Search by category can be changed to search invalid month

[floridsleeves]

In admin 'Posts' page, the search category can be selected by drop down menu:

However, client-side attackers can bypass items in drop down menu by intercepting the 'cat' field in request, which will return data from other non-existing categories.

Possible fix is to add server-side checks to this field to validate whether the category is valid.

[hasanuzzamanshamim]

Related to #55923