option 'show All entries' is broken

[gbolo]

the datatables js client sends a length of -1 when selecting the "All" entries option. Our server API does not support that and responds like:

Error 1064: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '-1' at line 1{
  "draw": 0,
  "recordsTotal": 15,
  "recordsFiltered": 0,
  "data": null
}